I’m a little surprised at the pushback, too. I read lots of things on other devices where it’s easy to select a new word and see its meaning. And it seems like such an incredibly cheap feature to implement and support that I can’t imaging not having it.
Just because a device can do something, doesn't mean it should. The RM team, for all their faults, were intentionally minimalist, and did not set out to make their device an e-reader. Complaining that the non-e-reader reads e-books poorly is just missing the focus of the device. It's about imitating paper but without futzing around with reams of actual paper.
Anyway, I don't really see why reading a PDF means one wouldn't need a dictionary. A lot of PDFs - that I read anyway - tend to be more technical stuff; looking stuff up is helpful there too.