No harm.
I can't say DH or I really understand how to "play," and I'm not sure our DD knows all the real rules either.
But she and her friends have been collecting, trading and organizing them for a couple of years now. (First and second grade). It's harmless stuff.
I actually think they use some math sometimes to figure out which combinations of cards would together make a fair trade. And there's lots of categorizing, classifying and discussing pros and cons of various cards. Not explicitly educational, but developmentally appropriate.
I remember similarly detailed and endless conversations with my friends about stickers. My parents thought it was bizarre how we assigned different (and shifting) values to seemingly random stickers, but we did it in order to trade them. At least for DD and her friends (both boys and girls), Pokemon is similar in a way.
There are some dads in our friend group who take it all WAY more seriously. That part is a little funny to me. The first and second graders are pretty chill about it.
P.S. Ask around for hand me downs from friends with older kids. We got a huge stash of cards from friends' middle and high schoolers, who have long outgrown their collections.
