I haven't exactly looked at the stats for the High Allagan weapons (edit: Actually, I checked while typing this up), but it is possible that on some if not most that the secondary stats on them may not be ideal. For example the H.A. Grimoire of Casting has +46 Crit (only 2 more than a capped Novus) and +32 Spell Speed.

Spell Speed isn't that helpful as a Summoner, but if you were to cap the +31 Det on your Novus I think your over all DPS might actually increase with the Novus rather than the High Allagan weapon. Needs testing to be sure, I suppose. But I know that Det is going to increase your damage, and that Spell speed really isn't. Not as a Summoner, when a lot of our abilities are already instant cast.