As I've said, any leak from these is also possibly a sign of a small vacuum/pressure leak. It's a good idea to replace them.
My driver's side was missing, so that was a huge difference, but I decided to also replace the passenger side.
At the time I was watching the short term fuel trim like a hawk (was fighting the bank 1 lean code), so I definitely saw before/after changes to the trim. Before the passenger side replacement, the trim was hovering in the appropriate range, but only on the positive side. After replacement, I started to see negative numbers too, It became centered on zero..