As far as player-controlled craft go, I agree completely that Ions are fun, effective, and an all-around good fighter-tool for disabling craft.
However, all of your canonical arguments could just as easily cover tractor-beams as well - in terms of film canon (cannons!) we see a tractor beam used to capture a ship once - and that's the Falcon (admittedly, by the DSI, with it's planet-sized reactor core & such, but still...), and an ion cannon used to disable - but _not_ capture a ship once, at Hoth - and that one's planetside too, possibly using the same huge generator that powered the Hoth energy shield.
This would seem to suggest to me that both techniques are inefficient at best in terms of energy expended per vessel disabled, and that at best, several hits on an opponent of similar size (fighter to fighter, capship to unshielded capship, death star to death star... (Okay, side note, someone needs to do an accurate Death Star I model so I can set up a DS vs DS battle...))
Assuming such inefficiency is the case, as I'm going to do here for the heck of it, we could be seeing the reason that the ISD was pursuing the Corvette with limited turbolaser fire rather than ion cannons.
Or maybe they just didn't want to scramble the data before they captured the ship so they'd know they had the right data files...