When is it okay to lose a caster level?

[ARandomGod]

It is an oft repeated mantra on this board that losing caster levels is greatly disadvantageous for an ambitous spellcaster. So the question is:

Is it ever okay to lase a caster level?

Yes. And I'll respond to the titular question too:
"When is it okay to lose a caster level?"
Obviously, when the benifits outweigh the 'price'.

The PrC Eldritch Knight suggests that a feat + a fighter's BAB is worth -1 caster level.

That it's completely not worth. However, as it also grants you access to the rest of the PrC, you can consider it a buy in price and look at the whole PrC to determine what you're getting. In that case you're getting a feat and *several* BAB... It's still only marginally worth going for the class. Generally you have to give up two caster levels, needing to take a level of fighter to qualify
(I hear that there's a Prc in the Draconomicon, Dragonslayer, which will let you qualify for Eldritch Knight without loseing another caster level
Req: 5 BAB, Dodge, Iron Will, Tumble 2 Ranks.
Good fort, will, full bab, caster level every other level starting at first, and of course, the martial weapons proficiency.)

Planetouched (aasimar/tiefling) suggest that +2 to caster stat, plus some resistances are worth it.

There you'd be wrong to. That's completely not worth it.

And it would seem to be worth losing a few levels to qualify for other PrCs.

Sometimes

I suspect (know) that there are people who say that these are not worthwhile trades, even in a less-that-ideally-optimized concept of a character.

But what trades do you find worthwhile/reasonable?

One example I am thinking of is a one-level intial dip in barbarian before beginning as a sorcerer. Even though the sorcerer is a level behind in progression anyway, I think for real adventuring it is hard to beat the hitpoint bonus, saves, speed bonus, for someone who wants to be a long-lived sorcerer.

Thoughts? Other examples?

Kobold Stew.

A one level dip is much less 'costly' for the spellcaster than a two level dip is. Spellcasting levels go up exponentially in power, so one less level is less than half the power of two spellcasting levels... and you have to get something REALLY good for three, etcetera (Example, a properly done arcane trickster in 3.5 costs three, but it's worth it (once again, if done properly).

I personally wouldn't dip into the barb as a sorc... not without a lot more payoff than a few measly hit points. If you want extra hit points, take 'False Life' as a second level spell known. You'll get more than you would from first level barb anyhow. You should be using your spells to get all those things, and it'll work better. What do you need speed for? You have DDoor. And if you really need the speed, cast a spell. Saves? OK, nothing really there to help. Take an appropriate familiar?