Str/Dex/Int(I'll call these primaries) will reduce the number of turns related skills go on CD, reduce AP costs, further increase damage/potency, and increase chance to apply control/damage over time effects.
And even if you have plenty of one of those primaries, you're better off with more points in speed/con/per (probably in that order) to use more of the skills those attributes affect than putting points into another primary.
Why go for speed if you can use multiple haste sources with a second primary?
Why go for con if haste increase your max AP and healing becomes a non-issue because of several sources for healing, etc
Your so sure that your numbers add up right? Because I am not so sure that speed is useful at all for glass cannons for example. You hit 20 AP per turn anyway quite easy. Furthermore there there is a decent amount of enemies that have above 100% elemental resistances or which have very high armor values and magic resistances. There was a fight when my mages started to stab an enemy with a pickaxe because it was the most sensible to do. (After that I gave them much more buffs, so they could stay quite useful in the future, which worked like a charm later in the game)
But I think that instead of giving them buffs, giving them some strenght and men-at-arms 5 would have worked out just fine too. Permanent inspire in your party sounds btw like a nice compensation, those two points to all attributes alone compensate a lot for hybrids.
Mixing a dex/rouge/ranger sounds not so good with elemental mages, but witchcraft and some level 1 elemental sounds like a very good combination with those skills. Air/Water for example are very useful even with just 1 point in it, if you skilled up int at least somewhat.
One-trick Ponies have one problem in DOS: They become none-trick ponies against some enemies.
