i'm a full time spy player and i do agree, the spy is kind of underpowered, but if any of these were to be changed to the spy's side, he would probably be overpowered. The spy class is a challenging class, it's what makes it fun, and makes me keep using it. After going through rockets, flames, minigun fire and all that, then pulling off a chain stab, a massive burst of excitement runs through me.
The harder the conflict, the more glorious the triumph.
Even though at times I've rage quit over getting constantly killed over things such as; coincidental spy checks, or hits, really absurd spy guesses, etc. I still would like to keep the spy as is.