It's not that terrible. The problem won't present itself until an advenced, high level stage of the game and probably a couple of moves at most will be left unmapped since there are so few of them.
Largely depends on the number of available talent points. I've been poking around guessing at a 35 total point cap, which is likely 13-16 talents total. If you can only get 25 points, which is pretty low (level cap of 20 or so), you'll get 9-12 active talents. At a bare minimum 1-4 will be left unmapped. If the cap goes up to 35, you would expect half of your abilities to be unmapped, with a number excess abilities taken for the sake of skipping over them and getting to the passives underneath.
When it comes to this whole "8 abilities sucks!" deal, I'm reminded of the adage "restrictions breed creativity."
Just a lazy retelling of "necessity is the mother of invention". Sure, when you have a tool for a situation, you don't have to find another way. But in the limits of a a game, you're already making that call when you select a talent at all. There's an opportunity cost to taking the talent outright. When you limit the number of talents you can bring, you don't really find a way, you just use the same ability more and rely more heavily on certain overpowered abilities. You had 3 abilities in ME3MP, but that doesn't mean people found creative solutions to jack. They just relied more heavily on singular abilities. I didn't go into a Drellguard platinum solo thinking "what can I do with these 3 abilities?" No, I came in with trained reflexes to mashing Charge and grenades and not even speccing Pull at all. No creativity, just skill. People didn't figure out creative ways to run a Gethfiltrator; they just spammed 1-3-shoot-repeat until everything fell over. Creativity was never the strong point of ME3MP except in the creative ways people found to break the AI and horde design (e.g. lazy platinum farming strategies). It was all about efficiency and skill.
Another way of saying it is that since you're not offering any new way approach a problem, you're just putting a low level cap of about 15 on everybody and telling them to be more
efficient (not creative!) with their picks.
//EDIT: For another refutation of creativity or challenge, look at how ability effects are crammed together. Glyph of Paralysis was shoveled into a fire glyph not because it offered challenge but because you have to make those 8 abilities pay rent in vastly more situations than they otherwise could. Making abilities useful in more situations doesn't increase challenge, it decreases challenge. You never have to go far for the cure for what ails you. I still believe the decision was made to make the game perfectly playable without any pausing, which is why I assumed they made the call because of MP (can't pause in that mode). Because a radial menu means a combat pause, disabling it means you can design encounters for the number of abilities used without pausing and not offer an advantage to others who pause regularly.
You can change which abilities are active in the field as long as you aren't in combat.
People won't for reasons I've already discussed. The option might as well not even exist, honestly. I'm going to end up looking over trees, picking my abilities, and then spending all excess points past those 8 skipping over talents and jamming passives. I'm pretty sure they abilities are going to be dodge/KEx3/barrier/heal up front, which leaves me 2 slots at best.