I thought normal nirnroots DO respawn, which is one reason this problem is so annoying; the source of the problem is never permanently removed.
They didn't in Oblivion, which is why many assumed they wouldn't in Skyrim (for some, in some games, they don't respawn). In my game, they stack and the chore-like pressure of having to pick them is either annoying or downright frustrating (depending upon my mood and RP-game circumstances).
Fortunately I caught onto the glowstack, improper enemy weapon and arrow cleanup early enough in my game that by playing janitor and weed hunter I've managed to keep my save file relatively small (500'ish hrs., lvl 49 and 12mb). From the many reports in the xbox forum, save files approaching 18mb or higher seem to be when the crashes/freezes, inability to load areas, and frequent lags tend to start up and escalate.
I love this game, but their (apparent) inattention to this issue really peeves me. I gave up numerous games in Oblivion because of the A-Bomb they never fixed. If every subsequent playthru requires the janitor/weed-picking chores I'm enduring now because it never gets fixed, I'll be severly disappointed.