I am wildly curious who is making this claim. Martone is certainly no Selke candidate and he isn't a high-energy puck-retriever, so he's like Smith in that regard, but simply by being bigger and stronger and faster and knowing where the defensive zone is (love Will, but he's very bad defensively) he is competent defensively. I'd project him as average defensively for a winger.
I have no idea who is bringing up notorious defensive zero Brady Tkachuk in this thread as someone we should be pursuing, but if you don't like Martone I don't see how you can like Brady. Brady is slow, poor defensively, and has effort and consistency issues. Martone has no flaws that Brady doesn't either.
If all of your players are hard-nosed puck-hounds like Eklund and Celebrini, you have a very boring and simplistic team. Toffoli is usually the guy lurking high in the zone while Celebrini and Eklund forecheck and that combo of guys works well. Replace Toffoli with Martone and it's even better.