it is not always possible or desirable to use the management port. In the example given by Matthias in his question on top, at least another port on the VPN gateway would be needed. If more than one switch is needed at the branch, at least one additional switch is needed for the out-of-band management. And how do you manage the out-of-band management?
Enterasys added source interface/IP configuration to the SecureStacks EOS for this use case (a small site, LAN routing on LAN switch instead of provider router, remote management needed). Enterasys implemented the default management IP for CoreFlow EOS to keep the existing networks working despite the unified IP stack of EOS version 7 and later.
EOS customers might want to replace some of their switches with EXOS devices, but they can only do so if the EXOS switch can be made to work similarly to the EOS switch.
Cisco IOS implements source interface/IP configuration seemingly since forever. Routers and multilayer switches are usually managed through a loopback interface. I have not seen one customer using the out-of-band management port on Cisco Catalyst 2k or 3k switches. Many Cisco routers do not have a management port at all.
I have seen EXOS customers use the management port, but I have not yet seen a network exclusively using out-of-band management, there have always been some switches configured for in-band management (not just the out-of-band management network devices, which need to be managed in-band as well). Many EXOS customer installations do not use the management port at all.