Good insight into this aspect of the industry. I guess this is why the move to in-game/in-engine cutscenes has become common place, I imagine it's cheaper, faster, and you can make changes relatively quick to the content.
That's actually more complicated than that...
Ingame cutscenes require a separate division, so either acquiring additional resources or taking away existing ones. Some game's engines are not entirely suited for cutscenes, particularly strategy games for anything character driven; or FPS games for anything really large scale.
Costs aren't that clear any more either. What you need to realize is that a LOT of the work in CG cutscenes has to be done even in realtime - previz, mocap, animation, lighting, sound, and so on. The staff will require software and hardware and office space and salaries; and on top of that they'd require lots of tools that would have to be developed for that single purpose and thus their cost couldn't be amortized over multiple projects. Even a lot of the existing game assets can be reused nowadays, sometimes with some detailing work but sometimes even without that. It's not like CGI is 10x more expensive nowadays.
There's also a quality ceiling for ingame stuff - complex character deformations and effects, special FX like fire and smoke and water, scope, etc. Cheating can also be more complicated (the stuff we can do with 3D matte paintings for example). This however also can work backwards - the client may actually not want to get cinematics looking that much better than the ingame content.
Also look at the actual releases - last year the Halo 2 anniversary release had almost an entire hour of CGI, COD AW had 20 minutes and now the zombie DLCs also add about 2-3 minutes per episode. Although I can't really tell you why these decisions were made and if this is going to continue; but that Locke trailer and the hints in the Halo MC Collection suggest that we're going to get more of this in Halo 5, at least.
Oh and the fans seem to still respond well to this stuff, they like these CG movies (at least the majority) - lots of fan sites maintain a repository of them so people can download and watch them even when not playing the game.