If the bit that holds the microphone is also adjustable, it gives you more range of positioning, and allows you to move the mic around without having to move the base around too. If the mic is fixed with a moulded-plastic clip so you have to move the boom around to position it, it's much less convenient. Especially for micing a guitar, where you often want the stand to one side of the player, the boom going across in front of the player, and the mic pointing at right angles to the boom.

Graham.