If you intend to do compositing, Motion satisfies your motion graphics needs, and you're good at learning new applications, I'd say go for it.
I jumped from After Effects to Motion/Shake recently and have been extremely, extremely disappointed with Motion. It is buggy to the point that I can't believe anyone accomplishes real-world work with it. You get a particle system looking right, then apply a filter to it (blur or 3D or ...) and suddenly it jitters around on the screen. You apply GPU-based filters and other filters suddenly don't have any effect, or you end up seeing frames from a different point in time. My limited use of Motion before this was mostly positive, but now I've reconciled myself to using Motion to generate titles, particle systems, etc, then doing the bulk of the work in FCP.
And that's why I mention Motion and your motion graphics needs. Shake barely does text, doesn't have particle systems, and has a timeline that feels very different from FCP, Motion or AE. It is super-flexible and it's customizable on a scale that FCP/Motion/AE/et al can't imagine, but it's not After Effects.
Shake is very fast and powerful for what it does: compositing, etc. It's an incredible app, really. But if you expect to use it like After Effects or Motion on steroids (i.e. motion graphics), you'll really experience the "cracking a nut with a sledgehammer" syndrome.
Shake handles interlacing fine and the results look good. In fact, you can set up your timeline so that you can move in time increments of fields to see each individual field if you want. In the FileIn node you indicate that the clip is interlaced, and when you go to render (if not before) you check an Interlaced box in the Global tab to output interlaced footage. (It's like Motion in that regard. Personally, I want an Interlaced checkbox in the FileOut that works independently of a checkbox for on-screen display, but that's me.)
I also recently moved to HDV footage and it works fine with that as well. (Though I still haven't gotten my head around how I'd take HDV footage and output a DV widescreen movie if that's what I want to do. Shake is so flexible in that regard that it's actually a little hard to figure out, coming from the AE world.)
I see no way to use B-splines for rotoscoping, though I have to say that rotoscoping with its Bezier curves is a bit different from what I'm used to in other apps. As you move one point, it seems to adjust the adjacent points' handles instead of leaving twisted curves you can get if you move a point and all the handles stay the same. (In Commotion, you can obviously use multiple rotoscope masks where it makes sense, and that helps a LOT in using Beziers instead of trying to have one super-flexible path.)
I'm new to Shake, so take it for what it's worth...