Yes, using the same code everywhere is cool, but if it takes longer to load, is less responsive, and has a worse experience, you have saved yourself time, but made something worse.
I imagine it like a 3D printed plastic hammer. If I want a cheap toy for my kids maybe it's great, but if I'm putting a new roof on my house I want a real hammer, forged in steel, etc. You know, so that I can have the best tool for driving nails into wood. I don't care that it would be cheaper/faster to make a plastic hammer, I care about a tool that is great.
I know that properly executed and with enough processing power, maybe performance is a mostly non-issue, but in my experience mobile JS dev isn't there yet and it's not remarkably better this year than last.
I hope this Firefox OS thing works out for Mozilla, but until the perf issues are solved, it's not going to be too exciting.