Odd, I was thinking the same about you.
10-level Prestige Classes could gain +4 to caster level for a feat.
Why limit them to 10th level only anyway?
So that at epic levels you can become even better at them.
Base classes can normally go up to 20th level. Specialized classes are limited to 10 levels or less, pre-epic. If less, no epic progression is allowed.
Someone who takes 5 levels of cleric then 10 levels of assassin (way past the starting point requirements of the mystic theurge which he qualified for as a 3rd level assassin) and then adds 5 levels of MT to up his cleric level to 10 and his assassin caster level to 15 (no new spells, mind you; just caster level increases) gains what? Other than getting 5th level cleric spells for 20th level, he is getting a caster level of 19 for a 10-level PrC before he can take his first epic assassin level (where his caster level will be 20, almost equivalent to the steady straight 21st level wizard with a CL of 21, or the 16 wiz/5 archmage with CL 22).
One of the few perks of an 11th level assassin is the ability to raise caster level above 10 (or with one certain feat, above 14). It cheapens the ability to allow it to be gotten so easily at so early a level. The epic system doesn't really reward players with enough new baubles to warrent starting to hand them out at 16th level because someone saw a way to bend the system. Compared to pre-epic, post epic play has barren choices for specialization (Prcs, feats, spell-like abilities, supernatural abilities, spells, magic items, companions/pets/familiars/golems/allies/etc.).
If you would have 11th level assassins with a CL of 20 in your game, so be it. Not in mine. It takes a lot longer to achieve such a high CL for a minor spellcasting class IMC.
Ciao
Dave