You may very well be right, but what you're leaving out of the equation is important: Pocket Queries are customized per user. Each one is only useful to a single user. Even if all users customized their PQ to be identical, Geocaching.com is still pushing out each one individually.
An RSS feed, however, would be a single stream for everybody, with possible individual filters on either client or server side.
Presuming the interest in each is equal (I know, I know), a very strong argument could be made that the latter would have less of a footprint from the perspective of burdening the system. However, your point that Geocaching.com can control when this service runs (ex: during the slow hours of the overnight, leaving the site unburdened during daylight hours) would no doubt be compelling enough to stick with the former (ie: existing) option.
... but the equation isn't as simple as Push-is-less-than-Pull