Yea. A permanent reduction just wouldn't work and increases micromanaging of skills and tedium of gameplay around draining attacks. However, looking back at my idea, I think one issue with temporary draining might be how it would choose which skill to drain. If it was a specific skill, how would Crawl know? If it was a random skill, would that encourage people to just spam level 1 of every skill to decrease the chances of it draining something important?
Thats a little roadblock, methinks, to having skill decreasing spells. The only workaround I can think of is giving skills weight for which one draining would prefer, with only a bit of randomness involved around two skills of the same weight. That's a bit of work though to change something that already works... decently. Draining could be improved upon, but I'm not sure if that's the way to do it.
...or maybe a reduction of all skills equally. Heck, this isn't even the right forum. I cant think at this point