Josh, if I recall correctly, the reason why you aren't increasing every workout is because you're using dumbbells with too large of a jump between each weight.
That is one totally legitimate reason to just increase reps for a while until it gets easy, and then to increase weight and drop reps back down to a "starting" number of reps at the new weight.
However, training this way will move you between training strength (when you first make the jump to the new weight and can only do a small number of reps) to training endurance (when you have been at a weight for a while and can do many more reps), and back and forth as you progress.
Having fine control over the choice of weight that you will use in any particular workout is important to keep pushing one aspect of muscular development.
If you want to train strength, you should keep using a weight that you can just barely complete 4-6 repetitions with.
If you want to train muscular endurance, your weight increases should also be small, so that when you increase the weight, you can still do 12+ repetitions.