I didn't say it wasn't, although creating four rectangles (and four animations) plus bindings and property settings between them is a lot more work, but then we're talking on a rather minute-level, and for a component like a busy indicator, that sort of consideration doesn't matter. On the other hand, if you're working on something like Label, where a bad property binding can be the difference between 0.x ms and 2ms to create a single Label -- that sort of thing does matter when you're creating tens or hundreds of them in a typical application, or while panning.

I'm not drawing direct comparisons here, I'm just noting that performance is (or should always be) a concern when designing something. Otherwise you end up with compromises somewhere further down the line (precreating, caching, prestarting applications, or just being slow...)