Telegram also requires a phone, as far as I understand it.
But I agree that we should class them both as "phone messengers", intended to be SMS-replacements - that's how a lot of normies do most of their messaging (when they don't use Facebook Messenger - and I have talked with the engineer at Facebook who set up their facebookcorewwwi.onion service about the possibility of using Axolotl in the future).
It would be very beneficial if Signal could be decoupled from that requirement - both for desktops and for tablets (finally, it'd be fully Skype-killing!). There's a Signal desktop client in beta which works as a Chrome extension, if you have an Android phone. I have it. It's a start and it works, but it's really only a start.
But then, it would also be beneficial if we had better metadata protection for messengers - because that is also a very important problem (people are literally targeted for murder via communication metadata!) - and that is one neither Signal, Threema or Tox solve.
Secure messaging in the presence of global active attackers is an open research problem, and there are no very impressive comprehensive solutions yet. Ricochet and Pond are quite good in their own ways, but they still have shortcomings.
But if someone's using Telegram, or Kik, or LINE (sticker support looks like it's coming soon?), or Viber, any of those other phone messengers that are out there, of course SMS right now, then Signal would be a much better choice for them. It would also be a better choice than WhatsApp or iMessage. That's something we can improve today, for ourselves, and the less technically-literate. We can help people make choices that suck less, while we prepare something that doesn't suck at all.