I've sat and ruminated on this for awhile. I think that ChrisF's answer is indeed the right answer (saving one from one of the Community Managers to really say one way or the other).
On occasion, I do exactly this - use a down vote to try to cut down the amount of crap that is out there. I've even got a query on Data.SE (Cliff questions (v2)) to help find these questions... and there is still more crap out there.
These questions clutter search results and set bad examples - both for how a question should be asked and answered. On the bigger sites, this is likely fighting against the tide with a little bucket. On P.SE, I've got a bigger tool to use (the delete vote) and there are like minded people who actively act to try to present the best foot forward for the site (and there are also people who likely disagree with this stance - and there are avenues of recourse to address this).
The like minded people is something important to consider... This also appears to be something spilling over from Math.SE (you guys have some wonderful drama going on in your meta)... so lets look at that:
From Around how many questions get deleted in Stack Exchange and by who?
Site Name | Mathematics | Programmers
AllDeleted | 21419 | 9857
AuthorDeleted | 10663 | 1748
SpamOffensiveDeleted | 102 | 46
Deleted10K | 218 | 851
DeletedCommunity | 10028 | 4955
DeletedModerators | 378 | 2559
Your 10ks appear to be very, very lacking in the "lets clean up the crap", your mods aren't being 'evil'. For some reason you want that 10,028 count for community deleted to be even lower (even at that, its still only 2x of what P.SE's is). You've got 10x the questions of P.SE, 10x the questions/day of P.SE and 1/4 the 10k delete votes.
I would like to say that I would very much like to have the roomba scripts revokable delete with 10k undelete votes and not act upon questions that have been undeleted... but thats a different matter... that appears to have been silently addressed. Yeah Haney♦!
I also think that the roomba scripts should do more. Again, thats a different matter.
Failing getting a current statement from the community managers, the best I can do is find previous statements and cherry pick them. This is certainly subject to quote battles.
From A modest proposal for voting about controversial historical questions
Outside of duplicates, rare is the question that should be kept around, closed but not deleted. And we have locking for those. -- Shog9
If you think there are closed questions that should not be deleted, fix them. Make them good questions. Reopen them.
If they aren't you are going to find yourself wallowing in crap questions from history as new users keep asking "well, that is ok there..." and new answers are "You multiply before adding, so the middle term is zero, so the answer is 41" (and the like) because those are acceptable answers that have been up voted in the past and continue to display how the site is to be used.
Guess what: if your site is full of crappy questions, your site sucks - even if they're not highly-ranked by your own users, folks are finding them via Google, and that's where the vast majority of your readers are coming from. You can work to fix that - as painful as that process is - or you can bury your head in the sand and blame it on all of those stupid people from elsewhere. -- Shog9
Granted, thats in the context of hot questions... but you need to fix it too. You can burninate and blacklist the homework tag, and write a FAQ about it (heh, but nobody reads that) but that doesn't change what the site is about.
Trying to change the roomba scripts to delete less to preserve some rep of some people who got drive by up votes on questions is likely not the right way to get higher quality site content.
I say this as someone from a site that has learned this hard lesson and is continuing to try to improve its image. If Math.SE wants to be something other than MathHomework.SE, it likely needs to learn these lessons too. If this is the case, it is unfortunate that its gotten to the point where it is now before having to delete your crap and throw away those fingernail clipping quality old questions.
Yes, this answer may have gotten a little ranty.
Watching the 'Recently Deleted' in the 10k tools scroll by when the roomba runs on P.SE, I would hate to see what would happen if it was even more selective about what it deleted. I would likely be terrified of what Stack Overflow would be like in that case.