reason not use pfsense as transparent firewall?

view full story

http://serverfault.com – Our office is using a multi-wan router as dhcp/router/firewall. but now I want to put a pc with pfsense between that router and switch, so I can do more advanced firewall task like traffic monitor and content filtering. after reading I found that pfsense can be a ip-less transparent firewall, so I just have to plug the cables to both NIC and done. this solution looks easy and I dont have to change my actual network settings. but is there any disadvantage or reason not to use as transparent firewall? in transparent mode would I still be able to create rules? and when should I use pfsense as (HowTos)