Link Gun primary fire can be very useful!
You can keep your enemies away if you shoot in the right moment.
Example: Bridge on Necropolis.
I killed plenty of Snipers only with Link Gun.
Sure I also got killed sometimes, but I definitely killed more than I got killed.
Also:
If you shoot an enemy with the Shock Beam, he will find you quickly and you will not damage him that much.
If you make the same with the Link Gun, you will damage him more.
It takes more time until the Link Gun plasma bullets (sorry if the name is stupid) hit them, but once they hit him, he will constantly lose health.
Just think about Kargo.
It's often too late when enemies massively shoot the tank with Link Gun primary fire.
Sometimes I have the feeling:
The more you are far away from an enemy, the more the Link Gun could be deadly.
Plenty of plasma bullets are coming, not everyone's dream^^
And as Splitter said:
No weapon is useless in UT3
Greets