Yeah, but I'm a broke high school senior (1-man team), so I'm sort of forced to take advantage of free stuff like Bootstrap and the free year of Amazon EC2.

Bootstrap is definitely useful if you're just tinkering - it makes a minimum viable product look sort of legit ;)

But I would certainly invest in specialized developers to make a lighter-weight, in-house UI if I were really serious about a website.

If you really care about your customers instead of your product, and they are more than happy with the "professional" Bootstrap UI, you have no reason to mess with it.

What is always good to have is a designer or at least a developer with an eye for aesthetics, to use the Bootstrap elements in an elegant way. Otherwise, what would you "hack" about HTML/CSS? Either reimplementing Bootstrap, or playing with the latest and greatest that won't work in anything else but the latest Chrome (see the IE6/7 discussion).