The in-line tuners don't induce string twist, make for quick string changes (especially if using double ball end strings), and the quality ones seem inherently more stable (tuning wise) than traditional machine heads.
The tuners are also right at hand when trying to correct your tuning mid-song, while you're fretting a note...no awkward reach over to access your tuners.
Some headless designs (think Status) have a section of string beyond the nut, before termination (Status calls this a "bend well") where you can press on the string to bend harmonics or add vibrato, just like you would on a Jazz or P bass.
This is all in addition to the weight, balance, compactness, and playability factors already mentioned.
Aesthetics can be hit or miss, depending on your tastes, but there's no denying that the minimalist, new concept and material design of the Steinberger set the bass world on its ear.
Like every other aspect of bass (axes, amps, pedals, etc.) it's all what works for you.
