Well there are several advantages to BESM.
1: Easy to learn fast-baced rules that can have quite alot of detail.
2: Compatibility with every genere imaginable without needing new rulebooks/sourcebooks.
3: All you need is 2d6.
Of course, I find myself being a Silver Age Sentinels player. It uses the same system as BESM, but more complex.
Of course, the Tri-Stat system is WAY customizable, and if you're looking for what you could call the "ground floor" of BESM, and SAS, check out
TriStat dX Okay, that's enough pimping...