Who questioned the plausibility of the morph ball? You can clearly see the morph ball is the same height as her crouching in most games. Crouching on one knee without her head even tucked in. The suit would have to reposition, unfold, and possibly reshape some parts to a) fit and b) cover gaps to become spherical, but even in a passing glance you can see she doesn't need to be a contortionist, and you can see that the dimensions are easily plausible.
As for why: it may have been a programming shortcut initially, but a lot of the game was designed around it so that must have been decided early on: the video makes it sound like an afterthought, when it would have very likely been a part of early stages of development. The reason is irrelevant though, because it is a combat suit, and mobility is an obvious benefit. Slowly and noisily crawling into a hole while under fire isn't quite as useful as quickly rolling into it. Furthermore, the addition of spiderball in later games (such as Prime) demonstrate that her Chozo suit can hook into existing Chozo technology (monorail-like tracks)- given all the above uses within the game universe's context there's nothing really 'lame' or questionable about its presence. She can make herself smaller and less of a target without slowing down and also interface with existing systems (transport systems, unlocking mechanisms, upgrade and repair stations, etc).
In a discussion about the plausibility of the morph ball, I'm disappointed to see the one implausible attribute about it was not mentioned: the third person camera control. You can theorize that she doesn't get sick or disoriented because she's used to rolling, you can theorize the ball's movement is pushed with gyroscopic devices within the ball, but what can you say about how she actually navigates? This is really the only topic that throws the whole concept into question.
