For major subs, absolutely. Take /r/videos for example. What's to stop bots from uploading porn to /r/videos at breakneck pace? You need a small army of moderators to close the floodgates. It might not happen immediately, but eventually, there will be massive spam submitted to default subreddits with new moderators.

Moderation takes time and unless Reddit is going to pony up $$$ to pay mods to sweep in on short notice, then I suspect people who casually volunteer will quickly be overwhelmed.