Well technically TFC did both. The old engine didn't allow for filters to restrict a specific teams access. There were doors and sentries. Except on Avanti, Casbah, warpath and maybe dustbowl.. But the spawns were so far out of the way it wasn't worth taking the detour to access the spawn area's as the flag was normally just around the corner (atleast in comparison to where the spawn was).
The sentries were there on the off chance someone managed to get into the spawn whilst a friendly player opened the door unknowingly granting access to said someone.
At any rate an engineers sentry stood on it's own wouldn't make much sense.. other than for gameplay. I guess it makes as much sense as an invisible wall though.. A stand alone sentry in a spawn area would look out of place imo.
If an uber did manage to get into a spawn area for a while, it would be sensible perhaps to deactivate any spawning within that spawn room with some triggers etc etc.