There have been a number of review articles showing that when surgeons double glove, the most common event is that the outer glove gets perforated while the inner glove is left intact. Thus double gloving as compared to single gloving reduces the overall risk of you getting contaminated with something.
Now if you stab yourself in the hand with a 14 gauge needle, no it's not like the second glove will be a magical barrier. But that's not the most common event. There are many times when gloves will develop a small tear from traction, or you just barely scrape it with a needle or instrument. In these cases the inner glove does keep you from getting exposed.
As a surgery resident, I would say that EVERY student should double glove EVERY time.
The only reason not to double glove is if you really think you need some extra tactile feedback for a technically complex case (such as sewing a vascular anastomosis), and even then I think it's a stupid decision. I always double glove. I always make my students double glove.
The argument that its "wasteful" to use a second glove when you aren't really doing much is stupid. The amount of waste in hospitals is already absurd and adding one pair of gloves to the mix is hardly making a difference. And it's never a waste in my mind to do everything you can to ensure your personal safety.
And just because you aren't really doing much doesn't mean you arent at risk. Surgeons may set a needle down right in front of you without noticing where your hands are. Things get dropped/tossed/thrown. I've seen students have needle sticks before, and personally had one as an M3, and it is never a fun thing to deal with.