I look at it as extra cooking space in basically the same footprint. I have a Shirley pit that has a charcoal basket for the warmer for dedicated direct cooks and a baffle from the firebox to the warmer that I just used to cook a bunch of chicken. Still learning how to manage temperature zones between the warmer and main chamber but I like having the versatility.
Plus as mentioned you can put a bunch of splits up there if you're cooking in the main chamber so you always have already source of wood ready to ignite.
Rob Dog BBQ
Shirley Fabrication 24x65" Cabinet model trailer pit with warmer