It's a gamble. In most cases, you won't need the warranty — but in the cases where you do, you'd be sorry to not have it, because repairs can be expensive. If most cameras needed warranty repair, the manufacturer would go out of business; it's generally the case that quality assurance is pretty good out of the box and most cameras are buy-and-forget.
There are some notable exceptions; it doesn't take much looking on the Internet to discover complaints about problems. Generally these fall under:
- Something broken right away. On used equipment, the original purchaser should already have dealt with this. In my experience with consumer electronics in general, something showing up in the first few weeks is the most likely situation (although still much, much less likely than nothing going wrong at all, of course).
- Systematic defects in a particular model — ditto, but you can also search for the model name for known problems and then check if the camera you're looking at is affected.
- Something that wears out before its expected life. This is really the gamble you're taking. And, yes, it absolutely does happen. It probably won't, but it's pretty hard to actually estimate the odds, and no one else can determine if it's worth the risk to you.
And also, there are generally other things which manufacturers will do under warranty which add value. Most DSLR makers will perform at least one camera body and lens autofocus calibration under warranty, for example. Or, the warranty on my Pentax camera included one full cleaning and overall checkup.
So, those things add value — but, here's the catch. This lack is one of the reasons use equipment is cheaper. In theory, it should be priced in to the amount you pay for used equipment. So, this really comes down to a judgment call over whether a particular price for a particular used camera meets your acceptance of these and other factors. And we can't really answer that part for you.