Eldritch_Lord
Adventurer
If what I'm understanding of your houserule is correct, full BAB classes are still free to multiclass and dip like crazy; they still have full BAB in the end. But for the medium BAB classes, they basically get a double whammy: not only do they lose more BAB with each multiclass which already greatly hurt, they're also now sacrificing ever getting to full attack on a move. And I'm curious about the case of say...a Rogue x / Swashbuckler 4. At at least some levels, he'll have a higher BAB than a straight rogue. But if you have to have exactly the appropriate amount of BAB for your level and class...he still loses out on those levels. Maybe it doesn't work like that, but that's how I interpreted what you said.
Note I said those characters who have all their levels in a full BAB class; fighter/barbarian or hexblade/ranger yes, fighter/rogue or paladin/monk no. There are still ways for characters for less than full BAB to gain the ability (and relatively easily) it's just that they don't get it by default. Think of it like Swift Hunter or those other let-two-classes-work-well-together feats: If you go straight X you're fine, but if you want to add Y you might need to sink a feat or two in exchange.
Asha'man said:The problem with "everyone can move and full attack" is that under the 3e design philosophy, you'd have to extend the same courtesy to monsters. Which makes everything from trolls to true dragons a hell of a lot more dangerous. I've played a few high-level fights where monsters had various "move and full attack" tricks (Disciple of Demogorgon, quickened teleport etc) and the carnage was incredible. It becomes even more impossible than it already is to fight defensively or protect wounded characters.
...and once again the "full BAB for HD" rule saves the day.

You might say that it's no problem if fighter-type characters and monsters thrash everything in their path, because all casters have Save-or-die effects, and from level 1 at that. But firstly, I don't think making everything as powerful as the most powerful option is a satisfying form of balance (too swingy) and secondly, despite the hype, magic has its weaknesses. It runs out. Wizards are fragile. (The 17th level wizard in our campaign died every other session for a while(!)) There are ways to protect against magic. But swords don't run out, and there are precious few ways to protect against greatsword to the face. When a fighter-type gets off a full attack, that's a lot of damage at pretty much any level.
I'm not one of those who believes magic is utterly superior (though I will say that if your caster is dying more often than once every 4 or 5 sessions that it may be a playstyle issue, like trying to frontline instead of buff from the back). I started playing with 1e and 2e, where fighters were quite competitive and were superior in many circumstances because casters were much easier to interrupt and SoD-ing fighters was much harder. I use this rule to try to shift my games back to a 2e feel, not because of any perceived imbalanced; the fact that it does help even the odds at higher levels is just a bonus.