You know Joe, I was just thinking this same thing.
BAB works just like skills, in the way you resolve things. It goes up at a mandatory progression by class, whereas a given skill is voluntarily increased. That's about it. The only reason BAB is manadatory is because D&D is a class based system. Fighters get a different BAB progression than Rogues. if you made BAB a skill, you'd also be wise to eliminate the classes (fighter and rogue), and just hand out a pile of skill points. Fighter types would spend theirs on BAB, rogue types would spend theirs on thiefy stuff. The MAX skill ranks rule would keep things in line fairly well as far as individual skill levels go.
Let's assume that the Rogue's SP alottment is the right amount (8+int mod)* level (I think I got that right).
For D&D rogues, they generally have 8 skills they want to keep maxed at each level, generally ensuring they use up all their points. Stereotypically, they don't advance their BAB very much, nor do they get diversity in weapons.
Normally, fighters don't have much for skills they want to keep up, but they advance a lot in their weapons skills (and they have diversity of choice).
Design-wise, you'd want to present the 2 extremes in a SP based system, rogues and fighters, with the ability to choose a middle build that isn't optimal over the 2 extremes. To do that, you need to make sure there's lots of fighty skills to buy, to force the player to choose between roguey skills and fighty skills. You can't just have 1 skill called "Fighting" (well you could, but you'd be wise to implement a SP cost per rank of advancement, like the old S&P for 2E).
So basically, you'd need a variety of "fighter skills" to give the fighter types something to spend on. You could break the skills up by:
1 Skill per weapon (longsword skill, shortsword skill, etc). This would likely consume a limited # of points because, most folks only use 2-3 weapons on their PC anyway.
You could break the weapons into groupings (gee, simple, martial, exotic), or by more specific groupings (slings, bludgeons, bows, crossbows, throwing, fencing swords, polearms, long blades, short blades).
You could then make various groupings be "cross-class" if you want to keep the class system, but move combat into the Skills rules.
Since the RAW fighter gets 2+INT, and the RAW rogue gets 8+INT mod, you basically want to have 6 or more interesting fighter-centric skills that most fighters would actually want to take. Like I said, most fighters only use 2-3 weapons, so to min-max, they would want to max out on as few combat skills as possible, so they could buy more thief skills.
(Remember, I'm assuming all classes use the same SP allotment, basically, same as thief)
Remember, you don't want a PC that is really good at sneaky stuff AND fighting or he'll outshine the straight up fighter and the straight up thief. To be a nasty killer, you'd just need a longsword, heavy crossbow, dagger, and hide, move silently, and listen. That's only 6 skills (by my proposed skill concept), leaving plenty of SP for maximizing those skills into a lean mean 1st level killing machine.
Janx