It's sad but true that everyone who wants to do a *lot* of meriting (not just 21 merits to max out a weapon or so, but like 100+, 150+, 200+ to really max out lots of stuff) would probably be better off levelling a merit cannon princess job to 75 to earn merits for their first job. Well, unless the first job was already a merit cannon princess job (which does cover a third of the jobs - 6/18, BRD RDM WHM WAR MNK NIN - so this isn't terribly unlikely).
Plus you get job diversity this way, especially if the princess job of choice had a different party role than the previous job. (Tangential comment: There's more to be said of job diversity about someone who has, say, BLM PLD MNK WHM COR levelled, than about someone who has DRK DRG THF MNK WAR RNG SAM levelled, for example.)
Of course, there's nothing wrong with wanting to get the merits on the non-cannon jobs either, but it *is* foolish to expect similar exp/hr (or invite) rates. And if you're *not* interested in the merit cannon princess job in its own right, then levelling it to 75 is kind of a waste of time if the *only* reason is to earn merits.