I guess the people (including me) who use the oil cans with buds don't read this thread that often anymore; otherwise, someone would have answered you sooner.
Anyway, I use the oil cans with buds for the same reason as you mentioned. I also place a spare screen inside the can to help prevent leakage, using the blunt end of a felt pen that just fits inside the can to push the screen flush against the bottom.
Personally, I grind the buds as fine as possible before placing it loosely (not compacting it too much) inside the can. I also use an unbent paper clip to poke a hole through the middle of the packed can to create an air channel.
Maybe you can get better airflow and more vapour from placing the buds directly into the heating chamber, but I wouldn't know because I've never actually done that before--and I've never felt compelled to because I've had no problem getting the effects that I want from using the oil cans.
I also suspect that there's some combustion going on inside the can. I always get some small amount of what looks like black (or maybe it's just a really dark brown) particulates after emptying the can, but perhaps this is normal, with or without the can? This is the first vapourizer that I've ever owned, so I don't know if some amount of combustion is inevitable no matter which vapourizer you use. I'm not maxing out the temperature, either--it's usually at 190-195 C (or 374-383 F). I guess I can try lowering the temperature, but I'm a creature of habit and it's not big enough of a deal for me to change things (or maybe I'm just mistaken about the combustion thing).