Yes but creating each individual part, and skinning the bones, is no different than doing the same in some tweening program, then importing the animation.
There's more to it than that. You can dynamically tween between any stage in any animation to any other animation. That's prohibitively difficult with frame by frame animations. Also, the bone animation is perfectly smooth at any V-synced framerate, because it uses TimeDelta - again, using framed animations, it is nearly impossible to get a perfectly smooth display. Finally, you can timescale and the animation stays absolutely perfectly smooth, even at very slow timescales, again thanks to TimeDelta. That's simply impossible with framed animations, you'd end up with noticable jumps as it switched frames at long intervals.
In short, using the built in bone movement will have better display quality, and be able to do more (assuming it matches the features of what you want to do).