- Users with a reputation between 15 and 125 can flag a post as very low quality but cannot downvote.
- Users with a reputation that isn't much higher than 125 may want to avoid downvoting an answer unless the downvote is really useful. A downvote on an answer that's going to be deleted isn't really useful, since the score no longer matters when the answer is deleted. On the other hand, if the answer is deleted, the reputation cost for the downvote is refunded.
- Users who vote a lot may refrain from voting when it isn't really useful (see above), because that increases the likelihood of running out of votes for the day. It doesn't really help for this situation that the vote is refunded when the post is deleted, because unless that happens very quickly (unlikely outside of Stack Overflow), the day may be over, or the voter may not be around to vote more that day.
The advice for downvoting answers is “this answer is not useful”. This very strongly applies to any answer that's worth flagging as VLQ, so a downvote is justified. But it isn't mandatory, and to some extent it's redundant — deleting a post at -1 or -5 doesn't really matter. The only way in which I can see the score at deletion mattering is to convey a message to the author, which doesn't apply to drive-by crap shooters, and doesn't really work anyway since the score will depend on how fast the answer gets deleted, which in turn depends on how many users visit the review queue after the initial flag, what their reputation is, and whether a moderator comes by to cut the wait short.
Given that the downvote looks redundant, I won't blame a user who flags as VLQ without downvoting, even if the good reasons not to downvote don't apply. It isn't obvious that even though you've flagged an answer to be deleted, you should vote on it.
There is however a reason to downvote bad answers, including ones that will be deleted. Accounts that post a large proportion of bad answers get throttled, and the lower the score the worse an answer is considered to be, with deletion being an additional malus. So for accounts that are active, an extra downvote can be what finally quietens a persistent annoyance. For a probably well-meaning newbie who doesn't quite get it, there's no strong reason to force or avoid an extra downvote; a single bad post can be redeemed easily by a decent answer.