This has bothered me for years. Everything just looks so chunky and inelegant. The difference really hits home if you compare Rhythmbox and iTunes (OS X version, not Windows) side by side. Rhythmbox just appears to be wasting so much space.

Arguing that it makes the interface cleaner might seem correct in theory, but in practice it really doesn't seem to hold true. Especially when you compare to OS X, which has an extremely clean interface without any over padding of elements.