Because I sometimes like to listen to music on my Shure 530SEs (especially on the train, since they're sound isolating). Compression artifacts are much more noticeable on high fidelity headphones.
And as a side note, I've always kept space to under 1GB, across all 3 gens. I haven't ever seen a performance hit, though in fairness, I only have them over 1GB free while experimenting looking for the differences. Also, itunes recently added an option to fill all your free space with random songs- not a move I would expect them to make if "too full" lead to decreased performance.
