To answer why exploration for Mass Effect, we have to answer what exploration even is.
It isn't open world.
Ever since the popularity of the Elderscrolls series, many have gotten the idea that exploration = big open spaces. That's not entirely correct. Open worlds facilitate exploration, but aren't it's sole proprietor. While we might find wandering around a fully rendered environment to be more responsive and rewarding, exploration can be found in the tiniest of vignettes and the quaintest of conversations.
I know that within the context of this thread and the larger debate, we're probably defining exploration as "adventuring in larger environments which hold a combination of the main game and side content," but considering the topic on an abstract level is the only way to truly understand why Mass Effect needs exploration and why that exploration doesn't necessarily need to be on foot.
With that said, I want to discuss what the general benefits of exploration are.
Choice and Agency
This is the obvious benefit of exploration. Letting us do what we want, when we want. It lets us project ourselves onto the player character and feel directly connected to game world. It gives us variety and allows us to think intuitively. It adds replay value gives the player a real sense that the game they're playing is their game. Players should get as much agency as possible, but only when its to the game's benefit.
Forcing the player down a specific path may be necessary to plot development, and the act in itself can send a powerful thematic message. Opting for linearity can produce consistent quality and potentially greater depth.
Exploration needs to be a balance of the two sides, giving players enough agency to feel connected and engaged, but not to the point where focus is lost entirely and meaning gets dashed along the minutia of every choice.
Discovery
Let's face it, finding stuff is cool and in game design, it's an incredibly effective reward system and story telling device. For one, it makes the player feel like a super-sleuth. Every trinket he or she finds is theirs. They earned the right to see it by going off the beaten track turning over each individual stone. It can also hide the seams of a game, showing the player pertinent information without shoving them along an amusement park ride. Most importantly, discovery establishes a universe of interconnected objects and events which culminate in a broad, yet interesting lore.
So yes, exploration is some thing BioWare should strive for, so when I criticize open-world, know that it's because I want an effective game, not simply a linear one. I want BioWare to tighten their scope, because vast environments and aimless wandering aren't necessarily conducive to good exploration. Here's what I think the next Mass Effect should aim for:
Good Design Is Deliberate Design
Some may disagree, but I think choice purely for the sake of choice generally isn't good game design. No, choices don't always need to be life altering or directly related to plot or even make a modicum of sense, but they should have a good mechanical or thematic excuse.
By deliberate, I don't mean linear. Although those often get piled together, exploration doesn't have to be at odds with depth or deliberate game design. This video has a relatively painless (if long) explanation about why, but it boils down to one question "what do they eat?" In other words "why and how is this here?" Choice is an aspect to exploration, not an excuse. The conclusion to your argument can't be "because it gives the player more options," because that's how we get fetch quests.
Every environment, distraction, and element of gameplay should give back to the universe as a whole, if only in a small way.
Don't Make It Gamey
While respawning enemies and collectibles certainly give the player something to do, they can trivialize everything they represent. I don't want to shoot faceless mooks in yet another pointless camp while collecting yet more featureless bits of something useless. Boring or not, they're obvious videogame tropes and undermine any sense of credibility gained by the open environments. I should be fighting Derek and his gang of jerk wads who were so recently slumbering in their distinctively magenta colored encampment and collecting the audio diaries of Derek's poor father whose political background forced his entire family into a life of crime.
The same goes for any repetitive tasks. I'm willing to rescue a wounded quarian during a side mission, but if I'm asked to do the same exact thing somewhere else, I'll just get tired of playing the game. I realize certain design aspects are unavoidable so at the very least, try to hide the "gamey-ness" and give each bit of content a distinctive (hopefully fun) twist. I can't possibly stand more shameless fetch quests.
Everything Bows the Core Mechanics
Mass Effect is a third person shooter, and unless anything has changed recently, it should act like one. BioWare had a bit more slack with DA:I, where cover, mobility, and line of sight aren't very large concerns. At the very least, every combat environment should facilitate a good firefight. There are plenty of ways an open area can work with the core mechanics, but there's also plenty of ways it can get in the way. I don't want any fight to take place in a boring open field.
Don't Stretch Yourself Thin
Another leading cause of lame content. I'd rather have a puddle sized ocean than an ocean sized puddle. One of the first things I learned about game design was lilt the scope. BioWare's no indie studio, but they can fall into the same traps. Keep things keen, condensed, and deliberate when possible.
Don't Forget the Narrative
This should be obvious, but I'll say it anyway. BioWare's greatest asset is a good story and they need to use it. As I said earlier, exploration isn't directly at odds with a deliberate narrative, but it certainly is easier for details to get lost and scattered across a bigger environment.
So far, I don't think anything I've said directly condemns open-world gameplay. It's certainly a valid approach to game design, but not one I think Mass Effect would benefit from. Ultimately, I want to make it easy on BioWare. So far they haven't had a perfect track record with exploration, side missions, and fetch quests, and spreading their resources across a wider scope isn't going to fix that. I want Mass Effect to get the exploration it deserves, so I think BioWare needs to start with depth and pack as much detail as the can in a smaller, more forgiving landscape.
I do hope BioWare might expand eventually, but history has only proven how poorly BioWare scales. As I said, open up the world to exploration, but keep it small and keep it deliberate.