Well the Tevs aren't bad at sentry guns. The Zods themselves let them handle it. It has been said that one of the yet-unseen use for Tev cruisers was as potential minelayers, deploying clusters of sentry guns to defend strategical positions or cover retreats.
Those would be, however, support tools first and foremost. Sentry guns are only efficient in very specific circumstances, and easily destroyed. That holds true for both small sentries like Cerberus guns, and Mjolnirs.
Oh, absolutely. Like I said, it's just to buy time while the warships plot jumps and escape vectors, collect remote targeting data, etc. I get the feeling from canon FS1/FS2 that the sentry guns were always intended to be used in that fashion. They can't guard a depot by themselves, but they can prevent enemy fighters from popping up in the middle of the depot and going to town, maybe buy some time for a response team to launch.
Long-range anti-subsystem cannon turrets, torpedo platforms, Paveway/Stiletto missile launchers, EMP mines...I always felt like those kind of tactics were never adequately explored in the Freespace 'verse. When you're facing an overwhelming force like the Shivans, you've got to get creative. And sneaky. I think some very specific UEF technology could be useful for that sort of thing.