the X-Server is used to start the Pulseaudio server (that is used for the Bluetooth connection). As some may use Jivelite or other visualisations and therefor need the X-Server anyway, it is the easiest way to make this mandatory and have only one Pulseaudio instance.
The running X-Server should not make much of a difference on current Pi (2/3) setups – but yes, on Pi 1 it may take longer to start.
Support for multiple Bluetooth Adapters may not be too difficult, but the audioquality suffers from bad bluetooth connections and player synchronisation with Bluetooth is also not as good as with normal soundcards (delay between players due to BT-connection). It won’t be possible to stream bluetooth with the normal BT-Dongles through walls…
Multisqueeze for Multiroom audio is best used with the build-in audiochip (headphone jack on Pi), an additional audiocard like the AMP+ from hifiberry / iqaudio / justboom and maybe one BT-Speaker. With this you can stream to 2 or 3 rooms with just one device wich works great for me.