It's not because of MP. It's not because of consoles. It's because that's the design decisions they made for the games they are making.
These aren't unrelated.
Using the radial menu slows down play on the consoles. If XBox/PS4 controllers had five buttons instead of four then we'd be looking at ten hotkeys instead of eight.
BioWare didn't pull eight out of a hat; it's what you get with four buttons + L trigger toggle.
And yes, MP influenced this decision as well. The 4 abilities + 4 utility items is taken straight from ME 3 MP. Again, that's a system designed around four buttons + L trigger toggle. It's also much easier to balance classes and abilities when classes all have the same number of skills.
I will also assume that the game has mostly been tested with controllers, and having the PC interface be similar makes it easier to fine tune the difficulty of monsters and encounters. Despite what Mike says about having 32 abilities, normal difficulty assumes you'll be using your PC the majority of the time and not constantly pausing. Giving xbox PCs 8 skills and computer PCs 17 means you have to change the game in other ways. Previous BioWare games typically had some difference when it came to immunities and HP depending on platform.
I think limiting the PC to only 8 abilities probably will lead to a better experience... but not because it's tactical*. If DA:I were only a singleplayer PC game, I expect that, like DAO, I'd have a massive toolbar of abilities for my mage and a smaller one for my non-mages.
Of course, having a massive toolbar of abilities isn’t inherently good. I play TOR and the pure number of skills and buttons to click can be overwhelming. I have a Trooper (Vanguard) and a Bounty Hunter (Power Tech) – they’re the exact same class with different names/icons for the same skills, and when I switch from one to another, I usually have to take several minutes to rememorize what everything does.
The kb/m control scheme encourages questionable design at times. I am a PC gamer, but I can see how designing for a controller first has some positives.
If each class + specialization only had 8 abilities total, I’d consider that a more elegant solution. As is, this seems a sort of cludgy way to take a development system from DnD that was altered for a cRPG and now needs to have parity between console and PC systems while providing both smooth real-time combat AND strategic, pausing party-based combat. Oh, and now there's MP.
What I'm saying is: Dragon Age is a platypus.
* Guild Wars and Diablo III use a similar limited ability set, but they were designed from the ground up around that. In Diablo III, my monk has access to ALL monk abilities and all possible upgrades for them. Likewise, all classes have the same number of active and passive abilities.
At level 70, there's literally no difference between the abilities of any two monks - only what they choose to put at their toolbar at any moment. At level 30 in DA:I, two different mages could have different attributes, different skill trees they've sunk points into, and different specializations.