I may add that as a pilot group, this is the rule that you want. It is really meant for people that move on to the big 6 and things don't work out or they fail out of training. The company and union allows you to come back and hold your seniority, which is perfectly fine.
What you don't want as a pilot group is people leaving for a quick buck at some contract job, leaving for a Flag of convenience (or some other foreign airline), or laterally to another Regional for a quick upgrade or some other reason and comes back once they find out the grass is not always greener and immediately fall back into their seniority.
Those people should not continue to hold seniority over you while you stayed contributing to the company. The union and the company smartly put in provisions in the contract that prohibit this type of thing from happening.