Look at it this way... Saying there is no real benefit to being a theif is like saying ther is no benefit to being a Mage type after all you can just use a melee weapon and a bow for ranged and heal with potions! BUT it is an RPG and you play a character type and you play for style at least many people do. Of course it is a lot more open in regards to skills but the point is to just have fun and being a theif is fun for many people sure there are other ways to make money but it's like they say it's the journey not the destination just play a character you like!

The same can be said about real life...some people prefer to take something rather then work for it. I used to know someone whole stole what they can, because they can and it wasn't born out of necessity or desperation, they simply enjoyed stealing.

If my characters join the Thieves Guild, they do so for training. Same with The Companions or the College of Winterhold.

I see it as a "place for higher learning" with the skills they offer, like sneaking, lockpicking, 1-Handed, light armor, and such. My characters may do the quests in the Thieves Guild (once joined), or they may just use it as a base of operations and training. (I almost always do the quests, though )

The Thieves Guild, for me, is a stretch on my own morality, but I sacrifice that for gameplay. I enjoy being sneaky and defeating locks

It's a lot easier if you come to the game from Oblivion. In that game the TG was the charity supporting the beggars, and other poor. If you RP someone who knows that, and wonders what happened to the one in Riften, then the quest line makes some more sense.

You won't be joining to become a thief, so much as reforming them from inside. The quests stop short of where I think they could have gone, so maybe I'll have to create a mod to complete it.