Two reasons that I can think of. First, LR is not inert, so it will absorb medications, such as copper, making dosing difficult. So, if/when you go to medicate, you'd need to take the LR out (and you wouldn't want to put it back in your DT). Second, if you ever had ich or other parasites in your QT, you'd want to sterilize the QT and everything in it before using it again (at least I would). Doing that would put an end to any LR.
You can use things other than PVC for hiding spots, if you want, provided they're chemically inert. For example, I use some fake aquarium decorations that provide hiding places. I doubt the fish care, but it makes the QT a bit more interesting for me!
