They do extra damage (actually not bad against unshielded targets), they provide an excellent defence against high yield plasma torps and power siphons, they can be used to counter an enemy tractor beam, and they allow me to fly through mine fields without a care (came in handy on 'Hunting the Hunters').
They will also shove cannon users out of optimum range, which can make BoPs a bit easier to deal with.
They are not a 'must have', but I do find them very useful
Tachyon beam level 9 really helps me get the shields down, I used to have emergency power to shields but that didn't help much if the regen is down, so I bought the klingon engineering officer and he has engineering team 1 which adds 1800 health, which is pretty good and it restores about 20% shield regeneration. For tactical i user torpedo spread. Becasue I use disruptors instead of phasers so the beam overload doesn't help me at all.
I currently have Tactical Team I, Engineering Team II and Science Team II, the shared cooldown isn't that bad you can use two of the skills basically before the first skill has reset. So I open with Tactical when I go into comabt and then use Science if I need it followed by Engineering, then I can use Science again after that.