With the way epic destinies are made, only certain destinies count as levels for certain classes. This leads to the other destinies being extra poor choices for anyone casting spells with a level component (either damage dice or SR, or whatnot). Paladins have the opposite problem where as a melee their best destinies are the ones that specifically deny them class level based advancement.
In the same way that epic skills gives you +1 to all skills, I would suggest that epic levels also give you +1 to all your class levels (if you already have such class levels) in the same way that certain destinies do this for casting classes.
Why bother you ask?
Other than the way this affects the choice of destiny as outlined above, there are other problems that this fixes. As of right now the current destiny/caster level works well enough as the cap is only 25, and a destiny can give you +5 to caster level. But what happens when the cap is 30? Or 40? Will monsters just all of a sudden stop increasing in spell resistance? Will the devs make sure to bring in loot that compensates for this increase? With the way levels are, quick cap increases are quite likely as there is no new character stuff to whip up, just loot and dungeons.
Second, non-caster classes also have class-level dependant features but with no destiney that increases the class level. One example is the rogue assasinate, which is 10+rogue level+int. Sure the rogue destiny gives you a bonus to assasinate, but again with fortitude saves rising over time as the cap increases, what are the odds that Int items and the extra rare +DC assasinate items will keep up? Not bloody likely is my prediction. Sure there aren't that many non-casters with class-level dependant abilities, but nipping this in the bud would prevent such future abilities being worthless at cap before they are even created.
Personally I'd be happy enough if the non-caster classes got class levels from their destinies (rogue from shadowdancer at least, fighter/barbarian from dreadnought, etc), but I think that setting it up so that class level abilities are independant from the destiny you choose is a much better way to do things in the long run.
The only problem with changing things in this manner is that multi classes will gain extra benefits from splashed classes. This is minor though as splash class abilities that scale with character level tend to need to be near maxed out to be truly effective in difficult content. The one exception is perhaps the lay on hands ability, but even then the character level component of that is much less important than the charisma component.
Too long? Didn't read?
--Make epic levels each count as +1 to all your current class levels for the purposes of class-level dependant abilities.
--This makes destiny choices more flexible for caster classes.
--This allows class-level dependant abilities to properly scale as the cap rises.
--Very little down side.