Keeping wired and wireless separate was mainly a concern due to all of the broadcasts that wired traffic creates. Those broadcasts degrade wireless traffic. We do have very good broadcast suppression, so why keep them separate anymore? Answer: To separate your troubleshooting and security domains. You want to be able to quicly compare any problems you have on the wireless network with the wired network to diagnose problems quickly. In an Aruba network, you can apply roles to traffic, which can limit what specific types of users can do. If you mix those users with wired users, it will affect that security model. You also in many circumstances would want treat wired and wireless clients differently. Keeping them in different VLANs allows you to do that.
Windows prefers the interface that has the highest cost (output of the "route print" command). If your wired users only have a 100meg interface, Windows will prefer a wireless interface that negotiates at 200megs. If your users connect via a gigabit ethernet connection, it is much less likely that windows would prefer the wireless interface for traffic, even though it would still be connected and send broadcasts to that client.
Some Windows wireless drivers have an option to disable wireless when wired is connected.
When devices are dual-connected, at times wired traffic will "leak" to the wireless side and create a duplicate user in the user table on the WLC with the wired ip address for that user. You can deal with that by enabling "enforce dhcp" on the AAA profile, or by editing the validuser ACL to only allow clients requesting ip addresses from wireless subnets. http://community.arubanetworks.com/t5/Controller-Based-WLANs/What-is-validuser-ACL-and-its-uses/ta-p/178584