I see that mic stands with boom "arms" (separately or in a kit) and boom "poles" with mic/light stand holders are in two completely separate categories on B&H.
One is a Mic stand with a built in boom arm (Boom arm usually 30inches or so)
The other is a a stand (with something lie a boom buddy attached) you can put your boom pole in. This gives you a longer reach, however you will need sandbags depending on length of boom.
Is there any difference between a built in boom arm and a boom pole in one of these "holsters" (besides, obviously, that you can remove it)? Are there other reasons to use one or the other?
Mic stand booms used in the studio are made of steel, so are quite heavy if you have to hold it over your head for hours at a time. Production sound boom poles are made of light weight materials - aluminum, carbon fiber, and graphite fiber composites - so can be held overhead for long periods of time. Since these boom poles are what is used 99% of the time on film sets it is more convenient to use a holster on a "C" stand that is also commonly found on film sets. It makes a lot more sense to have a holster in your kit than carrying a heavy, bulky studio boom in your kit on the off chance you'll need it once or twice a month.
A boom arm on a mic stand gives you a horizontal reach of about 30-45 inches away from the vertical stand. A boom pole held in a boom buddy on something like a C-stand gives you 8 to 12 FEET of horizontal reach so it's easier to locate the stand itself out of frame and where it won't cast shadows. C-stands also extend vertically higher than most mic stands.
Mic stands have more locking options for the angle of the boom. You can have a boom arm lock at any crazy angle to stick it in wierd and difficult places. If you attach a small boom arm (12 inches or so) to a normal boom arm, you can pretty much do anything you need to do, like make it go around corners. Also, most boom arms have a much smaller diamater then boom poles because they aren't telescoping in the same way boom poles are.