The phone numbers can be called any number of times during the trainer's designated times, so as far as I'm concerned, it works almost as well as the Vs. Seeker, and you don't need to charge it - just leave the trainer's route and make a call. So, I'm okay without the Seeker being included.
As for the Pokewalker... Well, as a chainer, I can say that it would've been fun to see more chaining in HG/SS. However, I recognize the fact that most Pokemon you would be able to chain in HG/SS would be redundant from D/P/Pt. After all, almost every Pokemon that appears in grass already appears between those first three games. What would they do, start putting fish Pokemon in the grass for HG/SS?
So, really, D/P/Pt for chaining, HG/SS for nostalgia fun times. Just deal with it and switch between the games whenever you want to do something specific.
- Tyros