No, those people having their slides plated aren't wasting their money. Many of them actually are aware that the plating won't be as rust-resistant as the Tenifer. However, I'd just opine that 99.99999%, or somewhere close to that percentage, are having their pistols plated for aesthetic reasons.
I made the point previously that Tenifer is only a few microns deep; however, it also makes the steel surface extremely hard. I don't know how much blasting Tenifer can take before it's gone, but I do know that lots of people remove the black finish and the Tenifer protection is still there.