Yea, like Todd said, that's some sort of myth that floats around.
I have used the Reptisun 10.0 exclusively for everyone for over a year now, adults, hatchlings, everyone. I've read all the UV reading comparisons between the two common bulbs (Reptisun and Reptiglo) in the 5.0 and 10.0 versions through screen and done my own readings and there really isn't any competition, the Reptisun 10.0 is the better of the four.
And by better I don't mean astronomical, inhuman levels of UVB. But it is a couple times stronger, which still puts it (with a Solarmeter) at about 35 µW/cm² 12" away in a screened cage vs. about 15 µW/cm² from a Reptisun 5.0.
To put that in perspective, in the shade of a large tree outside you still read something in the range of 70-100 µW/cm² depending on conditions, and about 250 µW/cm² at noon in direct sunlight where I am located. So given those numbers (and given that these bulbs decline over time) I do not think that 30 is an unsafe reading (comparable to UV available outside before 8:30-9am, here), while I do think that 15 is already borderline minimal when brand new.
That said, the Arcadia bulbs are much better still. They output more to begin with and will last you a lot longer than 6 months.
A good cage should always have spots of thick plant cover anyway, so they can choose how much of anything to expose themselves to, but the 10.0 bulbs are not going to fry your chameleons by any means.