We were talking about this the other day at work as they are doing free flu vacs next week. I've never gotten the flu vaccine, I don't see the point- it's just another needle, and *touch wood* I've really only ever had bad colds, not the full on flu. Also you are right, it only "protects" against one or two strains of the flu, as the flu is a virus it mutates quite quickly so the particular virus you pick up, the flu vac potentially won't protect you from anyway... I've heard of quite a few people who got the flu right after getting the needle, so obviously they didn't have a great reaction to the vaccine!
You still might catch the strain of flu you were given the vaccine for, you just might not get it as bad as what you would have otherwise. Are you able to get the vaccine when you are pregnant? May be handy for some as you can't take cold & flu tablets when pg. I won't be getting it though, never have and probably never will.