Cisco Campus Network Design Basics

while reading Campus Wired LAN Technology Design Guide the paragraph below mentioned a seperate management vlan should not be configured for directly connect router or firewall. Can you explain why?

If the switch is the only switch at the site and is directly connected to a router or
firewall, do not configure a management VLAN. Instead, use the data VLAN for both
data and switch management. When you use this configuration, the next step requires
that you also configure the in-band management interface VLAN using the data VLAN
ID. For example:

interface vlan [data vlan]
 description In-band switch management using data VLAN
 ip address [ip address] [mask]
 no shutdown