I’m not sure whether it is always a good idea to post an alert. True, it is a little disheartening if you notice as a chaser that you have missed an activity. It is also true that (in particular on VHF, or when propagation is lacking) sometimes an alert generates more activity. But that’s primarily because very few people seem to bother listening on the VHF calling frequencies these days, and thus not a SOTA-specific problem.
On the other hand, if I feel like chasing summits, I rarely bother looking for alerts on the computer screen, but prefer to tune across the band and listen whether anybody is bold enough to call CQ from an interesting place The few times I have more or less spontaneously activated a summit, an alert might have helped to gather a few more QSOs, but I could never be sure when I would arrive on the summit, and whether it would have been possible to “get out” at all (the German low mountains often have heavily wooded summits, which can make VHF operation difficult). So, it may be better not to put up an alert (even if it is only a statement of something planned) to avoid frustration on the chaser side.
Furthermore, alerts often haven the side effect that they attract a lot of callers, similar to DX cluster spots, with ensuing QRM and general confusion, and to me, alerts (be they on SOTA watch or a DX cluster) also smell a little of “spoonfeeding” or “third-party assistance” … As I wrote above, they make the most sense on VHF, but only because of the low activity.