Hope im not too late here. I'm pretty new to telos, havent actually played it at all. Ive played a crap ton of tf2 though, and a fair amount of tribes, ns2, loadout, and some other fps games that featured sentries or turrets. Coming from that backround, its my opinion that sentries have no place in these kinds of fps, or in any objective based real time action game.
1 . Poor skill indexing
There are three kinds of skill in real time action games: Mechanical, tactical and strategic skill. Mechanical skill is any kind of action where the challenge isnt choosing the right option, but simply executing it (ie aim, timing tests, coordination). Tactical skill is choosing the right option based on information youre given as you are in the game (ie, reloading at the right time, choosing the right target, moving the right direction). Strategic skill is the decisions you make before the action starts (where to attack from, what weapons to choose)
Sentries in TF2 require almost none of these skills (at least before we factor in unlocks). They require no aim to drop and use, they create very simple tactical decisions (hit your sentry until you cant save it w/o dying), and have ZERO strategic skill after you decide to want to use an engie since optimal sentry locations dont change, ever.
This is obviously a problem because no matter what kind of competitive game you want to create, you want to make players feel like they are good at the game. And what is the easiest way to do that? Create tools that require/reward skill. To a new player, it could seem like tf2 engie requires a lot of skill to be effective. But to the veteran, we already know what optimal engie play is, and it lacks a lot of depth. Also the fact that the power of the sentry does not skill with player skill, so it will inevitably be strong or weak depending on your skill level.
Unlocks change this a bit, notably the wrangler. Making the decision between healing and shooting can take some skill to do right, and choosing the right targets to shoot takes some good game knowledge. But even then, we reach other problems with sentries.
2 . Discourages player interactions.
What is the purpose of setting up defenses? No, it's not to fool players to walk into them; that is not a viable long term goal. The real purpose is AREA DENIAL. In other words, keeping the enemy out of an area, usually the are you are in.
I will concede that area denial can be used in small spurts to create dynamic and fun gameplay by temporarily changing the combat area. You can see this with grenades, temporary aoe effects, and sometimes explosive spam. It can force players out of turtley positions, and make players create tactical decisions on the fly.
However static area denial does not lend itself to new combat experiences. Instead, it rewards players for camping, and discourages aggression from enemies. While you may argue that discouraging aggression is a strategic decision that a player would make, I would argue that such a mechanic has no place in a multiplayer game. The appeal of multiplayer is interacting with other real players and making decisions based on their decisions. Reducing player interactions actively works against the fun that the developer should be trying to create.
A successful successful sentry means the other team will never have a good opportunity to actually fight your team. A game where nobody actually fights doesn't sound like a lot of fun to me.
The only way I can see sentries as improving game is where it is too easy for the aggressor to just jump in and do whatever he wants. In the case of tribes, without sentries, its way to easy for an offense to jump in and get out with the flag at any time with little response from defense. But imo, sentries arent the solutions in that case, there are other ways to reduce the power of offense w/o having a feature that frustrates players.