So are you meaning a LAN type IP on the WAN interface or a public IP? If it is a LAN type IP on the MX WAN interface that should be fine. The use case I can think of for this is plugging in a DSL modem etc and having it hand out 192.168.0.x type addresses. All of your traffic shaping rules etc allow you to just specify WAN1 and WAN2 so the IP based routing wouldn't be an issue.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
If this was helpful click the Kudo button below
If my reply solved your issue, please mark it as a solution.