This lib (joosy) was built to satisfy your particular set of needs, and it seems that it does so well. Cheers to that.
You could easily add the simple helpers in underscore too, they're not complex, but the idea is you use a library so you can work at a higher level of abstraction. Why write your own array intersection method? Why write your own camelcase function? Sugar opens a lot of doors that are just a little much effort to normally be bothered with (and underscore does too, but to a lesser degree).
"…start extending things that weren't built that way?"
It's ok to extend the prototypes, so long as a) you own the objects and b) you only extend the prototypes from a single module (e.g. sugar). c) Don't extend Object.prototype (unless you're really brave). http://blip.tv/jsconf/jsconf2011-andrew-dupont-everything-is...
The major risk is in collisions, and if you follow those rules you reduce the chance of this occurring to a minimum. If you're worried about for-loops, the way sugar handles extending the object prototype makes this a non-issue. http://sugarjs.com/native
Please actually use a tool before you condemn it.