Man, there are a lot of theories in this thread.
Poplar is a hardwood, but at the softer end of the hardness scale. It is softer than mahogany. Sound-wise, that means plenty of low end, but minimal high end attack, giving it a warm, mellow sound with limited projection and definition. So yes, some ply shells have a poplar core to help provide that "vintage" sound.
Poplar is known as a utility hardwood. It's inexpensive, plentiful, and easy to work with. The biggest downside for drums, besides the limited tonal range, is that it's really not pretty. It generally has a grayish-tan color with greenish streaks and looks overall like something that has been dragged through a swamp and then dried. Stain can't rescue it, IMO.
I use poplar for prototype builds and tests of tooling setups. To my taste, it doesn't have the tonal range to make it useful as a solid wood, and I don't like mixing species (which to my ear is a compromise of tonal clarity rather than a best-of-both end result). But for those using low-frequency inner cores like luaun, gum, basswood, etc., poplar can be a good choice.