I agree with you as well, ideally your DMZ switches should be separate from your internal switching infrastructure such that it is physically impossible to get from a compromised DMZ system to somewhere on the inside network without going through the firewall. By trunking internal networks with DMZ networks on the same physical interface there is potentially a path from the DMZ to the inside network that does not involve the firewall, as in the switch itself. Read about VLAN Hopping and other VLAN-based Network Attacks for some more background in this area.
Now with a properly-configured switch these types of attacks should not be successful, but the key word here is "should". It is still PHYSICALLY possible if there is a zero-day exploit for the switch or a new VLAN attack technique discovered. I'll gladly take "physically impossible" since short of someone gaining physical access to your facilities (which is a whole different problem) there is no way the discovery of a new VLAN/switch exploit will help an attacker.
One could argue that a new vulnerability could be found for the firewall itself, but I'd wager that possibility is many orders of magnitude less likely on a security-oriented device like a firewall rather than a network-oriented device like a switch.
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