Urist Ottanenen has gone stark raving mad!
Derro have a reasonable chassis with their three monstrous humanoid HD, which are only slightly worse than class HD. Their ability scores are neat, with reasonable bonuses to dex and con, a huge bonus to charisma, and an equivalent penalty to wisdom. However, since no derro is going to need wisdom for anything, this works out in your favor.
Also interesting are the spell-like abilities. Daze will be useless soon, but Darkness at-will can be used in a surprisingly high number of ways (At Home In The Deep, Instinctive Darkness), Ghost Sound leaves a lot of room for a creative player, and Sound Burst is pretty good control for a 2nd-level spell.
Finally, there's the other abilities: sneak attack, natural armor, sunlight vulnerability, spell resistance and racial skill bonuses.
However, it should be obvious that derro deserve no LA despite all these abilities. Just imagine a simple desert kobold bard/rogue/warlock (which is, as one might note, quite unoptimized). The kobold can use darkness at-will (warlock invocation), has equal sneak attack damage, and can even cast Sound Burst (through Precocious Apprentice). It has more skill points than the derro, is more versatile in general, and can move beneath the sun without taking constitution damage. The derro will have SR and better ability scores, but I don't believe those abilities outclass the kobold's.
Now, let us take it a step further and imagine a character not resorting to outlandish dips and using a better race than kobold. It should be plain that such a character is stronger than the bard/rogue/warlock, and therefore stronger than the derro.
To put it simply: derro's deserve -0 LA.