I picked up the crossbar on the 7 when I was a kid -- I thought it looked cool -- but now I just think of it as a very useful thing to help differentiate my sevens and ones. That's probably a relatively recent addition, really, as mathematics evolved and things like 1-7 or Z-2 confusion became more and more common... though frustratingly, I haven't yet found a time period for when the crossbars started being used on sevens regularly. Hmph.
Anyway, the fact is no one sat down and worked out a numbers system all at once -- they evolved, like any other language or alphabet, and their evolution can be traced pretty clearly. Angles didn't enter into it. Though I can see why it came about -- people like there to be some sort of order to these things. :)