I would love this as well. This is currently one of the major deficiencies of multiclassing (right behind spellcasting). We do fractional BAB in some of our campaigns, so multiclassing two 3/4 classes doesn't totally screw you and it works out really well.
But, like others have said, they're certainly not going to introduce fractional BAB into D&D, because so many people just wouldn't be able to do the math. This is an unfortunate truth, but even your average D&Der (who I presume have a greater than average intelligence), can't add some simple fractions together. Or if they can, they bitch about it.
One thing that could work is to do the +2 +3 +4 thing and instead of having a possible +120 BAB at 30, you just use that number to look up a chart telling you what your actual BAB is (which would just be that number divided by 4). Unfortunately that's not very elegant.
I'd also like to see how they do saving throws. It was always annoying that multiclass characters got such huge save bonuses because of their +2s at multiple first levels.
What could work for that, is each class effectively giving you iron will, etc, and not having them overlap. So basically, fighter gives you "fortitude focus" which gives you +2 to fort saves. If you then took a level of cleric, it gives you "fortitude focus" and "will focus", but the fort focus wouldn't stack with that of the fighter. That solves the problem that some other solutions have that only give the bonus +2 at first character level, because then it matters in what order you took your classes, which is always annoying.
-Nate