I have read that disabling DHCP and using reserved IP addresses has some security benefit - dont see it myself.
Does anyone have any thoughts?
You are quite right - there are absolutely no benefits, security or otherwise, in disabling DHCP whatsoever.
Just wanted to point out that 'reserved IP addresses' are DHCP assigned, but the DHCP server is configured to always allocate the same, particular IP address to a particular device (more accurately the device with a specific MAC address). What I think you were talking about are static addresses which you set manually on each device - and are a real pain to manage!