That's the first and only time I've ever heard of a band hiring someone as a regular member of the band, but with a term length. It just seems bizarre.
With any other work situation, you're just an employee, so the gig could be assumed forever (until you quit or get fired or something) or have a set length. But in a band, the assumption is always that you're in the band permanently. This is the band now.
Note that this is different from hiring side musicians to play on an album, or for a specific tour. Those guys are clearly not regular members of the band, just helping fill things out. But when Dio joined Sabbath, was it widely known that it was just a temporary gig?