Isolation isn't primarily to stop DNS leaks, it just does that as well. It is primarily to prevent hackers from getting your real IP address. Also, spoofing MAC address can be useful in this situation as well, look at the FH attack their malware gathered MAC addresses to be able to identify the individual machine that went to FH. It isn't only a concern about MAC address leaking to the wireless router but also a concern about remote attackers hacking in and grabbing it.