Netgear Nighthawk M5 MR5200 WAN issue

Not to hijack the post, but I'm just looking for some clarity while setting up my new home network and would appreciate some feedback.

Currently I'm utilizing a Nighthawk M5 (Telstra) > Cisco IE 3000 switch.  I am using the Nighhawk as my WiFi router as we were living in a very small apartment, and utilising the switch for ethernet connections.

That has changed now as we have moved into a much larger house.  I'm looking to update the switch and add a few AP's, however I am greatly interested in adding an external router and using the IP Passthrough of the Nighthawk.  After reading this thread, I can see how much of an issue I'm about to have.

My question is, what are the main advantages of using another router?  Why do you not just use the Nighthawk M5?  I don't think I need a VPN or anything else fancy.  I do realise there are a few restrictions with the Nighhawk, like limited Port Forwarding ability.

Thanks in advance.