I think so because it's a lot more convenient. I don't bring my laptop with me everywhere, so if I want to check my SpaceHey during the day, I have to hope there's internet and log on a separate time (because it will log me out every time I leave safari). With a SpaceHey app, it will be a lot more convenient to check messages, friend requests, and replies to my blog posts.
That being said, I do see the other side. As user Eamon pointed out, the difficulty of finding SpaceHey is part of what makes it special. I also find that my time using SpaceHey at home feels more special, since it's something that I have to make the most of.
Overall though, I think the pros outweigh the cons, and there should be a SpaceHey app. It just makes the app so much easier for people to use, and users who prefer to use just the website would still be able to do so.