Another thing to consider is that if you level all your classes on one character, you will not be able to gear them all out to their full potential when you are level 50, because of how the currency you earn in dungeons is throttled with a weekly cap.

If you have your level 50 classes on different characters though, you can gear them all up at the same time - Given that you have enough time on your hands during the week, since the cap is per character.

More of a "hardcore" consideration though I guess, since it indeed takes a lot of time to run all the dungeons to take advantage of this.