Agreed, but specifically for the 319's and 321's (the 763's have a reasonable number of rouge plus/preferred seats). The 319's and 321's are 29" pitch, as opposed to the comparatively spacious 763's. And yes, I find with many of the 319 routes, the preferred seats get booked up well in advance, given that there's only ~8 economy seats with decent legroom on the 319 (bulkhead, plus two window seats in exit row). Seems like demand exceeds supply for those for the routes out of YYC, especially to LAS - I've got some flights booked with rouge in December on a 319, and the preferred seats are already booked. If I'm considering a rouge flight in a 319, I won't book it unless I can get a preferred seat, as I simply can't fit into the 29" pitch seats either, and I've had occasions where I've booked WS over AC, simply because AC had no seats with reasonable legroom available.

That being said, I'd expect it wouldn't be insignificant to add some extra seats, as they'd likely need to take out a row, and then they may have more preferred seats than demand.