They can effectively bottle up a choke point for longer than a frag or plasma. I'm not entirely sure why they only let you have one in your loadout, though, considering that while they probably do far more damage overall the "real" amount of damage you're going to be daeling with them is much smaller.

I think at one point they were going to have an EMP effect as well, which I think would have been an interesting move and made them more effective--essentially they could have served as a weaker power drain + trip mine that way.