It's always good to get the perspective of the designer.
I tend to agree with you about the cleric; the super fast experience progression offsets a degree of slowing the spell progression. I think the issue is more of expectation, and (old-time) gamers knowing how those tables have worked for 30+ years.
Here's an example that lays out my concern about the base attack progression. (James, this example isn't so much for your benefit, I'm sure you live and breath this stuff, but rather for folks that might have only skimmed the rules and haven't chucked any dice or seen how it plays at the table). For the example, take an 8th level party and throw them up against a classic monster like red dragon. The standard red dragon (AC -1) would be AC 22 in LOTFP terms.
Our 8th level fighters would be +9 to hit, I'll assume they have a strength bonus and some +2 weapons by 8th level, putting them at +12 - they need a 10 or better, 55% chance to hit, good odds; as we know, fighters are the backbone of the party.
Every other class would be a +1; assuming some +2 items as well, they'd need a 19-20 to hit. For the cleric and magic user, they have better things to do than melee. The specialist is probably trying to backstab at +4 (+2, and +2 for sneak attack).
The demi-humans, on the other hand, can no longer carry their weight if they can only hit on a 19+, since they're no better at fighting than magic users. The WFRP rules have some nifty combat options that fighters, dwarves and elves can all use, so the "press" option provides an additional +2 at the cost of some defense.
One obvious solution (and one I've followed from time to time) is to normalize AC so that most monsters will fall into a 'realistic' AC range - extreme negative armor classes from other systems get pushed down in the AC 20 or worse range.
Another solution is to hand out tissues for the crying. It's a humanocentric game, sorry you picked a demihuman. (That one wasn't popular at the table when I suggested it).
My players have been petitioning that I give the demihumans a partial attack bonus progression - not as good as fighters, but something that lets a dwarf (fighter) attack slighly better than a magic-user or cleric of equal level. Like maybe at level 4 it becomes a +2 bonus, a +3 bonus at level 6, etc. (Basically half of the fighter's bonus, rounded down, minimum of +1).
Just thinking out loud at this point - wanted to see how it's worked out with other folks.