Can it be? A MM3 monster that's actually playable?

Petals are in a weird place where they officially aren't playable, but do have a cohort LA that not infrequently has the (cohort) bit waived. It's not hard to see why: Petals are arguably more playable than a number of creatures that did receive official LAs (krenshars, anyone?).

A petal's chassis is pretty awesome. +10 dexterity, +8 charisma, +4 intelligence and +4 constitution are adjustments many casters and skillmonkeys would kill to have. 60 ft. (good) fly speed doesn't just offer incredible mobility, it also outspeeds most PCs and monsters by a significant amount. Free Weapon Finesse and DR 5/cold iron only sweeten the deal (thought exercise: what's the last time you faced a foe with cold iron weaponry in D&D?).

Finally there's the petal's signature ability: Sleep Songs. The single-singer variant is pretty weak and amounts to at-will supernatural Lullaby. However, multiple petals working together can instead produce Sleep effects.

I think the current LA of +2 works fine for petals, though I could see arguments for +3. They are mobile and make for a good caster chassis, but their frailty (especially versus AoE effects or strong ranged attacks) means they go down easily. A party with two petals gains a rather potent save-or-lose, but the overlap of having two petals in the same party means +2 LA is justified even then, in my opinion.

Very interested on hearing what people have to say on this one.