It's a simplistic answer that justifies handing out a minor league contract to every has-been who has delusional fantasy of making a comeback. WHATS TEH HARM? LOLZ!?
If Oil Can Boyd was really throwing 90 mph, or was showing an resemblence of the pitcher he was back in the 80's and 90's, then there would be no harm in signing him to a deal. In fact, a major league team would have probably given him a deal. It's far more likely that this guy is cooked, and is not even worthy of being signed to a minor league deal.
I don't know the contract language, but I'm pretty sure if he throws his arm out, he's entitled to medical treatment courtesy of the Red Sox. Why waste money on a nobody? Why parade this over-the-hill pitcher who wasn't cracking 85 mph on the radar gun in the independent league?
http://www.boston.com/sports/baseball/redsox/articles/2005/05/17/rusty_can_says_hes_returnable/
Looks like he hasn't got the message.