The core cast of Bleach initially consists of Ichigo and a few of his school friends. As I've already mentioned, the plot in this story is entirely passive; the heroes just want to be insignificant nobodies, but thems pesky gimmick villains keep dragging them out of their happy apathy. As a result, the core cast starts off rather unremarkable and has very stunted growth. This problem is further compounded by Ichigo's gimmicky growth.
A standard arc in Bleach starts something like this: something bad is happening and Ichigo has to put a stop to it. Several of his friends want to help and tag along. At the beginning of the arc, each friend is a little less powerful than Ichigo, but respectable in their own right. However, they soon get wildly left behind. Ichigo gets a few power boosts, the caliber of the enemies increase, etc... Soon enough, said "supporting cast members", who were perfectly fine fighting minions in the first few chapters of the arc, are wayyyyy out of their depth. By the end, they're always cheerleaders...and they're usually left far behind before the halfway point UNLESS they can manage some gimmicky power boosts of their own.
As a result, you've got a core cast with members that offered to help and should have been involved, but are utterly worthless by the end. They're actually dead weight--less than worthless.
However, the plot doesn't REALLY work with a cast of one (Ichigo). SOOOO, the writer has Ichigo pick up allies and friends as he boosts in power throughout the arcs, usually following the "adopt your defeat rival, whose loss to you has greatly improved their life" model. I've always found that one to be tenuous if it's used even once and Bleach's whole plot depends on it. So yes, you'll have a slew of supporting cast members, varying wildly in strength depending on when they were picked up. The later they join on, the stronger they are...and the more time before they become dead weight.
After the arc ends, all the important characters (core cast+a few additions from the supporters picked up) are magically boosted to Ichigo's level-1 again and a new enemy attacks. Rinse and repeat.
As an end result, you've got a core cast whose job it is to fall by the wayside. The supporting cast, temporary allies, are usually far more interesting because they can actually do something. What's more, they often have motivations of their bloody own (Shinigami have a REASON to get stronger--they get promoted and their lives actually matter).