The game has a dozen little ways to gain XP as well. Each class has a hunting or gathering journal which will give small xp rewards. These add up after time.

Another thought is aesthetics.
Do you want to stare at the same character leveling every class? As a rule, I only play male characters as my main. However there are time I do enjoy leveling a female character just because its not the same thing. Certain classes just look better for females in my opinion. I rather play a WM as a female character than a male. Its not because of gender stereotypes, I just think it looks better. Same for Bard. I like female Bards better as well.

It boils down to, do I want to spend a few dollars more a month and have access to all 8 characters or do I want one super character that has access to everthing I need.