There are many attacks that can be launched from Layer 2 that would provide credentials to a user that hasn't authenticated to the corporate domain ...yet.
For example, Responder is capable of spoofing LLMNR and NBNS requests even without being connected to the domain -based on the way NBNS (and subsequently LLMNR) were designed. They are enabled by default in most environments and IT needs to explicitly disable them (which is usually not the case). In that case, without MAC filtering a rogue device may be connected to the network via the meeting room jack and obtain network level access, from their there they could perform a variety of spoofing attacks. ARP poisoning is a related attack that could also be performed.
Additionally, abuse of the corporate Internet may also occur if just any device is able to connect to the network and receive Internet access.