As others have mentioned, removing the bottom not only gives more clearance for a 3" deep charcoal basket but also limits the restriction of air flow, (As other have shared based on their experiences with 12" vs 9" pans for use as heat diffuser. the 12" diffuser signification impacted the ability to maintain good consistent temps.
The only advantage I can see in simply drilling holes is the pot can be used to transport things when not in use but using the steamer insert creates a base for that use.
It is much easier to drill one hole and then use tin snips and cut a 12 inch diameter circle out of the bottom. ( I traced the bottom of steamer insert to get my diameter.)
You can always drill the holes first do a few test cooks and if you find the need to cut out the base then you can always do that later.
Remember...Don't remove "the entire bottom" as you need to keep the curved based to maintain structural integrity of the steamer pot. leave about 1/2 to 1" lip on bottom of pot
Enjoy the project..