I personally don't feel like forcing someone to watch something by default is needed. Announcements (the major ones at least) are already sent out via a newsletter about a day after they are announced.
Did you just refer to yourself as "the OP"? It still seems like unnecessary 'forcing' on the hand of the developers. I've gone months without updating a piece of software and being alerted hasn't at all helped to do anything. Plus, since that feature isn't default in XenForo, it would have to be coded.
I remember when xenforo held that discount for RM's had they been able to watch announcements then yes. This would have also benefited regular active users who at the time so that would be a benefit and a pro. Drawback is they would need to switch to unwatch if they are active.
How many time does a thread go overlooked and maybe a comment mentioned in announcements? Alot as seen through xenforo's history. But I guess this is down to the devs if they want to apply forced watching on announcements.