hm, not sure this thread is properly titled, lol. first of all, i'm one of the few who can't read rowling: i find her books just too... too.... anyway, i've seen the movies. to me, it's not so much as a flaw in her magick system as opposed to the logic.
i think we know the real reason she (apparently) does this, and that's to tell a new story. her and george lucas should get together and make the most logically flawed movie ever made. there also may be certain skill levels a young would-be magician has to attain, such as magickly picking complicated locks requires a better spell. is there something limiting about the truth serum that makes it unuseable able, such as the ingredients are very rare or the incantation part is dangerous? (this personally is an intriguing thing i'm borrowing from 'army of darkness': a spell whose last word is so complicated, that to mess it up means instand destruction. if it works, however, nirvana or something, the secrets to the universe are revealed. a better way to 'kill' potter off, rather having him going down with voldemort, he uses this spell to defeat him then rather drifts off into the cosmos.... bleh. you're right, that sucks.)
where was i? oh, yeah. this obviously just isn't related to rowling's books, but stories in general where it's like, 'why don't they use such-and-such from the last movie/book here? would make things a whole lot easier!' a flaw on the author's part? for storytelling? honestly, would *would* you attack the vampire lair at night when you could wait til dawn and burn the damn house down? guess that makes too much sense. would it work? you betcha. exciting? not unless you're making s'mores.
something else i think is worth noting: most people aren't writers, therefore don't have the critical nature we do to half make a novel a study of style, plot, etc..
i've always wondered why potter, being so prone to adventure, doesn't master a few key spells that should come in pretty damn handy, ones perhaps beyond what his adversary would expect of his schooling. call me crazy, but if i could afford it, you betcha i'm gonna have a spare wand tucked into my sock (which could have interesting effects). something like a spare wand and a slick fire spell would have come in handy in that spider cave, if i remember the scene right.
not just rowling, it's a whole bunch of other books, usually series or franchises. it's just when you have to come up with something bigger and better and fresh, the 'basics' often fall by the wayside because they're not even interesting anymore.