[stormkeep]

You're 3 reasons cover it for both myself and several other people I know who have multiples of the same class except for one other point:

D.) Multiple specs without the inconvenience and cost of respecing.

Until recently there was no dual spec system and until Sunwell, money didn't grow on trees, so someone who liked multiple specs but didn't have a lot of money would sometimes just level another (or two) of their favorite class.

It's why I leveled multiple warriors. As a tank i was always broke from repairs, so i couldn't respec every other day to have some arms fun in pvp. This is also the reason that answers your final "but even so.." paragraph.

Not really valid anymore though...but then again, I would not level more than one of a class anymore at this point.

I know another person who has three priests because she kept changing her mind on what race she wanted to be...so there's another possible explanation. And some people who have two of the same class did so because they love the class, but when Draenei were added they wanted to be one of those instead of what they had originally rolled.