Just curious... I want to get it but it seems impossible without the thieves guild, and I'd rather not join them for roleplaying purposes. I know I could do without the one word but still I want to get it. I've heard I can enter through the back entrance but is it even possible?

Nope. You also cannot get full-power Flame Breath without joining the Companions either. And you can't get the full Ice Form shout without joining the College of Winterhold, nor can you finish the Forbidden Legend quest.

That's something that bugs me about this game: it's like you're required to join all four guilds.

Getting all three words is impossible without joining the Thieves Guild
I have tried entering some of the areas, but the game will not let it happen.

Damn, my friend told me I could glitch my way through... damn shame.

Not actually true, If parrthurnax is teaching you the second word to the flame breath shout and you spend a soul to unlock it before he unlocks it for you. He will instead unlock the last word. Its a bug but a way around joining the companions.

The Main Quest requires you to... enter the College.

Not actually true. You can get the Elderscroll directly out of Helgan. (if you can somehow survive the Falmer and other enemies).

Don't forget Slow Time. You can only get that through the Civil War quest.

I'm talking about the thieves guild...

And how many people are going to figure out where the Elder Scroll is without going to a game guide or previous experience?

*Anyone who explores the northern area of the map*

If its your first time through the game you shouldn't shun experiences you haven't tried yet so it doesn't matter. In any event the number of College Quests required is still zero. You can gain enterance and go directly to the Library get the information and then never return.

yes actualy you can, though you will have to do some clever tricks, but yes you can get inside and get the word and get back out, without triggering the cutscene at all and without getting stuck.