If you're fixing software, you're either fixing your own bugs or someone else's bugs. There's bug tracking software; these metrics are visible. If you're fixing your own code, you're also adding functionality (presumably you wrote the code you're fixing). If you're helping to fix other people's code, they will notice. People talk.
I've personally seen almost an entire team disbanded, the work outsourced. The people kept? The guys who fixed things, who made sure it shipped at the end.
The thing about being the guy who fixes software, is that no one will hear about it if you are doing a good job at it.