I think it really depends on the time-frame and acts involved. If you're having sex with someone and using even a non-porous toy, you're going to be at risk if you're sharing.
Other than that, the risk of bacterial infection is the biggest concern - but I wouldn't want to take any chances, personally. If this was an issue for me, I'd look up the specific STD and learn about its ability to live outside the body and what is required to kill it. But even then, I think I would rather just get my own.