This depends on some things:
1. The shape of your tank -- Hex tanks, and other tanks that are high have less surface area, thus there is less oxygen in the water. So if your tank has one of these odder shapes it may be a wise decision to purchase an air stone.
2. The amount of water agitation. The more water agitation more oxygen in your water. So if your filter creates minimal water agitation, it also might be a good idea to purchase one.
Replacing them -- They last quite a while when you see a significant decrease in bubbles it my be a good idea to replace them.