That sounds a bit farfetched to me, but I'm not a warrior. Would that really give a significant advantage? Then what about warriors running a conjure build (those conjures were around in 2005, weren't they?)? The 'no damage skills' argument sounds more likely to me, that just leaves DF as the odd one out instead of insp&heal&prot. But as you said earlier, anyway it doesn't make a lot of sense...
Support lines don't have a conjure spell.
Understand it was a decision made before launch even. A PvP game with secondary professions. Anet was very aware of there being potential issues when developing that system.
Its about attribute investment. If you invest 9 ranks in air magic, a damage line, then its not any different than a warrior investing into marksmanship and equipping a bow on swap. But we are talking secondaries like healing prayers where a warrior could deal the same auto attack damage as an air wand while having a lot of sustain. Yes wanding isn't exactly "rofl op damage" but it isn't nothing either. Back then some warriors in PvP arenas were engaging fights with vampiric bows before the clash, no marksmanship.
In gw2 anet had to relearn this lesson with the elite specialization for ranger, the druid. A very support specialization and its new equipable weapon type, ironically to this convo, was staff. It was very support /healing oriented and its auto attack was relatively weak. But even with it being a weak damage attack it had measurable attrition. They ended up nerfing this pathetic damage auto attack several times to address this. I believe this is something the 2005 anet devs were mindful of.
ESO has a similar thing with its restoration staff. The restoration staff does terrible damage and unlike the destruction staff (ice/fire/lightning) its auto attack damage has no damage improvements through its weapon line.