Well first of all marty is kinda pointless unless the server is overrun and you can't get away OR you are just super noob and you hang around for it to blow up. Every once in a while i'll run into a room after some p90 fool has gotten owned and run through his marty blast. Oh well.
I agree with greg on this one. Marty just doesn't boost your killing ability. It's a crutch that gets kills for those who are really nooby, but once you graduate from that level of playing, marty limits you. It takes a very valuable perk slot that could be used for deep impact. Slot 2 for me is always deep impact. If i am in a small game and the rest of the people are using dead silence, then i'll swap deep impact out for it. Really, the best perks are the ones that allow your killcount to increase with your skills. Deep impact is by far my favorite perk (also the one that gets me in trouble
).
If you can't dodge marty you got some skills to build.