For my 2 cents, I couldn't see myself dropping Glabrezu below LA +3 in my own games, for the reasons already given by others; bearing in mind I'm on the extremely harsh end of assigning LA, so I may be overestimating.

I also would like to point out Unholy Blight isn't as bad as all that IMHO. I would put it on a par (or just a fraction below) with unlimited Fireballs. The damage cap is lower (except against good Outsiders), and it sucks against non-good opponents, but it has a (minor) rider effect, and isn't subject to the very common Resist Fire X that so many creatures have. And lets face it, most of what a Demon fights is going to be good aligned in most games. YMMV.