F.B. 101:
Unless you have holes rusted / corroded THROUGH the flavorizer bars, or are missing major chunks out of the edges, then they should still be OK.
Some people tend to wear them out faster than others - may have to do with moisture in your area and how much salt / acids you get dripping on them from what you cook.
The flavorizer bars serve (2) primary functions:
#1. They vaporize meat juices and some fat as it drips onto them and slides into the catch tray & pan at the bottom. This is what produces the "char-broiled" flavor within a gas grill.
#2. They reduce flare-ups (grease fires) by keeping your burners out of direct exposure to dripping fat. (This can STILL happen if you cook a lot of fatty stuff and enough grease accumulates in the bottom of the grill.)
As long as they are structurally sound, they will fulfill this purpose just fine. Once they become perforated - then you'll be getting flare-ups and it's time to replace them.
If you're getting uneven heat, take a look at your burners running at high, medium, and low settings with the flavorizer bars removed (works better in lowish light). My guess is the burners may need cleaning. I would remove them and clean the buggers out, then re-install and compare your results.