Why can't the transporter be a weapon?

[Bad Thoughts]

Like I've said before: transport fifty photon torpedoes right outside the shield radius and see how well the shields deal with that.

If they hold out, do it again.

Beam, rinse, repeat.

In all the time you had your shields down, an enemy would be able to launch several volleys before you could drop the piano (that is too say, beam torpedoes outside their shields). That definitely would be a Wile E Coyote moment.

 

[Anwar]

Well, yeah. You have to usually drop your own shields to do the transporter attack, which usually makes you dead in a space fight before you can do anything.

The Borg were able to teleport through shields in their first appearance but never after that.