Is there a benefit in leveling a new class compared to rolling an alternative char?

[TheTen]

Cross-class skills are really not optional for a majority of classes/jobs, so you are expected to at least level most classes on one character to some degree.
Furthermore, you cannot send mail between alts... if you want to gear an alt with your main, you will have to have both of them in the same FC and use the FC chest, or ask a friend to play intermediary for you.

Beyond that, however, nothing speaks against making alts... you can play the starter questline for a different city (something that is not possible on one character), you may enjoy the look of one race doing certain skills more than others (Lalas makes everything look cute, male Roe make everything look slow and heavy, etc.), and let's not forget... you will have all quests available again.

Yes, you get a 50% kill EXP bonus if you level another class on one character, but that's not an advantage... it's compensation for not having the main storyline missions and whatever side quests you have done available anymore.

[tocsin]

also, on the pro side of alt classes, desynthesis can not be maxed for all skills on a single character, so that is probably one of the few end game bonuses for having multiple alts.

[ruskie]

if you want to gear an alt with your main, you will have to have both of them in the same FC and use the FC chest, or ask a friend to play intermediary for you.

The other thing is you can't really gear an alt with anything other than non spirit bonded crafted gear and a few minor dungeon drops. Everything else is untradable.