It also has a mental link with its summoner, and doesn't need to interpret a statement the same way a defense lawyer would.

It also follows ordered to the literal letter, it says so in the book. To the point of faceplanting into a ward if the target it's following passes through. It will then sit there until it expires.

I'm pretty sure "Watch the door" will get an "oh boy! oh boy! Master will be so proud!" result.

I'm not so sure. The book describes them as 'clever' about carrying out tasks right before saying they take things very literally. Seems somewhat contradictory.

They watch the door in a very clever fashion, to the exclusion of all else, exactly as ordered.
An F8 Earth Spirit might be very inventive about the ways in which it could fulfill a command like "make sure this man's life expires before your summoning does." "Watch the door," however, leaves no need for intelligence. A camera does that.

They aren't described as cameras though, but small dogs. They do have the capacity for some reasoning.

I dunno, watchers don't seem worth bothering enough with to worry about semantics getting the way of using them effectively considering they are barely useful to start with.

They're perfectly useful as long as they're used right.

Well let me clarify: compared to previous editions.

And considering how you work astral space and stealth at your table I guess.

They no longer fit the fluff of what they are supposed to be. The homogenization of spirits, the removal of domains, the overuse of extended tests made the search power to damn good. With domains and a single opposed test search was not the universal find power and the watcher still had a role to play and it had a unique and strong method of finding people. Now while you can find a use for a watcher it is just a crappy spirit, but hey you can have them up and a spirit without binding. I wish the watcher had retained its unique method of tracking and the search power was not an extended test, just a single test that takes X hours.