In my current town (strange community of crows), we had a big debate on it, and we decided that because we had plumbing, the watch tower is not that important since we have a kill team go out with a marked path everyday to clean out the renewed zone to make it safe for the scavengers anyway.
There is really no purpose for scavengers to go wandering off aimlessly beyond the first few days. Instead, we spent our upgrade on more important things, namely search tower, defensive focus, workshop, and upgradable wall.
The only time I'd see that the watchtower is useful is if your town has no scouts and you are having trouble finding the last few buildings on your map or you don't have plumbing/search towers.
Updating in response to Thechoyce's comment:
Upgrading the watchtower to level 4 or 5 to take advantage of the AP savings requires 4-5 days of not upgrading something else.
First, let me say that there is no need for builders to scavenge next to town, by day 12 we already had enough raw wood/metal to build every building, our limiting factor (probably for most towns as well) is N&B, depleted zones don't yield those.
Lets run some quick numbers using my current town:
We have roughly 12 full time scavengers (both heros and none heros), and 18 builders.
For watchtower upgrade 4, that saves us 12 ap per day. 24 for upgrade 5
Assuming we went straight for watchtower from day one, and we are currently at day 15 (we will see no benefit until day 4), we will save:
For watchtower upgrade 4:
12*11 days = 132 ap
For watchtower upgrade 5:
11 + 24*10 days = 251 ap
Here is what we would be potentially giving up:
Defensive Focus (from 5 - 1): we currently have ~100 defensive items in the bank, this would cost us 600 defence, which would mean we would have died last night, so definitely not. Also, there is absolutely nothing you can build for 251 ap that yields 600 defence.
Search Tower (from 5 - 1): The entire map has been depleted since day 5 on, and there would be no way we would have as much defensive items and weapons without the search tower, so that's out again.
Workshop (from 3- 1) & Upgradable wall (from 3 - 1): 71 defence: 2 upgrades on the workshop saves us 14% of all building ap, just adding up the buildings we've completed since then (roughly estimated at a cost of 1000 ap, false town + mega pump being the big ones), we saved 140 ap, so its a wash with WT4, but that's with only 2 upgrades into workshop as compared to 4 in watchtower. However, add in the 71 defence from the two upgrades to the wall, it tips the scales in favor of spending it on these upgrades instead of the watchtower.
Finally, we are currently fast approaching the point where the only things we can build require nuts and bolts (of which we are short by ~25 on), all the AP savings in the world can't do anything if we can't build anything. I'm not saying the watchtower is bad, and I have no way of knowing how much more we can find if we did extend our expeditions from 18 ap out to 20 ap, but from the point of view of this town, it certainly isn't worth the upgrades.