With some WP/BP themes having many small js and css files, the loading speed benefits from combining can be huge.
I was having real problems with both CSS and JS (404 and 500 errors) after trying to combine and minify with W3TC on http://virtualcrowds.org using BuddyPress default theme + the 3 column child theme, so I un-minified and un-combined everything and checked the original CSS files with the http://jigsaw.w3.org/css-validator/ W3C CSS Checker and this is what I found [http://jigsaw.w3.org/css-validator/validator?uri=http://virtualcrowds.org/&profile=css21&usermedium=all&warning=2&lang=en].
I’d had many problems combining multiple CSS files by hand on this WP3/BP site, and no wonder, as the verifier identified 128 Errors and 289 Warnings. They had no visible effect when separate files, but once combined caused obvious issues.
So just out of curiosity, I pointed the checker on http://buddypress.org and this is what I found [ http://jigsaw.w3.org/css-validator/validator?uri=http://buddypress.org/&profile=css21&usermedium=all&warning=2&lang=en, thats 130 Errors and 654 Warnings.
Now I am not a web developer, but I do know much is made of both WordPress and BuddyPress being standards based and driven.
So what chance has the little guy got if its all talk and no do?