As far as reasons go to level a character to 100, I might just have found the silliest one: I leveled my shadow priest to 100 because he had been in the mid-90's for so long, he ran out of bank space for the various level 100 gear reward tokens from follower missions. I hated the idea to vendor them for 5 gold instead of the full converted value, or missing out on some surprise when converting them to real items. But as conversion is only possible at level 100, I had to play the shadow priest some more.

The priest is probably still my least favorite character of the four level 100s I have now. But I found that towards the end of the 90's his performance got better. I'll see how he does against level 100 elite mobs when I have my barn upgraded to level 3, but he might just do okay.