pG disabled sentries on Hightower for a reason. Mostly for the reason that it cuts off a significant portion of a map and can be extremely difficult and annoying to even play around. Besides, the Hightower server is known for players playing with gimmicky playstyles, so it wouldn't be a surprise to see more than 3 sentries in an entire area with a bunch of people spy checking and forcing projectiles away.
Besides, the main reason the Hightower server exists (more or less) is for a TDM server. Just kills and skills.
If sentries were to be added, it would upset a lot of players as well as keep some players from wanting to do what they wanted to do because there's a level three sentry in one area of the map.
Sentries in casual Hightower are fine because:
1) Most players don't know much about the game so taking them down is a lot easier than say a comp engie joining the Hightower server and demolishing anything that comes his way.
2) There's a cart to defend and protect. Sentries are a great line of defence, but when a sentry is in a place that isn't defending something, it's more or less an annoyance to try to destroy it.
3) Mini-Sentries (albeit annoying) work for a quick defensive offence (if you know what I mean), but as soon as it's introduced to a casual Hightower server, it's either alive for way longer than it needs to be, or it's destroyed instantly because there's at least 2-4 decent players on the server.
Sentries on a TDM server aren't quite ok because:
1) Not that many players like the idea of something aiming for the engie, making engie just a class that can place a sentry down and run away without even trying. Mini-Sentries were balanced in GunMetal because of this idea. Making the Mini-Sentry spawn with 50 hp and slowly climb up to its full health after finishing its building.
2) Players would rather have the flow of gameplay be ever changing with no clear sign of an offensive line and a defensive line. Hightower does this best by spreading out the spawns and keeping the map extremely open for anyone to do their own thing.
3) Players hate having an objective in TDM servers. They'd rather just kill, kill, kill than trying to push a cart to the end. It's the main reason carts are disabled in the first place. So not having sentries would be not having an objective (in someone's words, yes).
TL;DR: There's a reason they're disabled in the first place.