I was cynical at first like you, thinking why bother. But when I tried it, turned out I was wrong and I actually had a pretty good experience!
The way I see it now, is that MPs aren't always in a good position to get close to the facts, so when you get in touch and tell them what you think.. you're actually giving them a huge gift.
It can actually be pretty effective, especially for state/local issues. For federal stuff, sure, might not be as good, but you'll at least get some satisfaction from getting an acknowledgement from their chief of staff or secretary.