Lazy, sure, but not always a worse experience for the end user.

Testing w/ Modernizr _will_ use client resources, and sometimes it's faster, cheaper, and less costly to make assumptions than run those tests.

Terrible case in point: "position:fixed". It's not supported in older iOS and IE6, and CSS fallbacks don't work. Testing for it w/ JS is too costly to be worth it, so UA checking is a better compromise. (That is, if it's mandatory to degrade well...)