It's not impractical at all. You clip it to your backpack. That's what I did in college, and if I wanted to ride a city bike in a big city, that's what I'd do there too. Every time I go into a walkable big city like NYC, I'm carrying my backpack with me since it's the most practical way to carry a bunch of stuff including my laptop. Clipping my bike helmet to it would be trivial.