If you consider that the PC's are designed to emulate D&D PC's, then besides being a character each of them would have a theoretical and imaginary "player" behind them.
Now, imagine the kind of player who would be playing a character like Belkar. He's gotta be getting pretty bored and frustrated. His only real contribution is melee. But he's been stuck with the mark of justice for a long time now and that seriously cramps his style. On top of that, he gets shown up by the rest of the party. Roy's a better warrior, so's Durkon when he calls upon Thor's might. Now even Elan is "horning in" on Belkar's territory with his Dashing Swordsman prestige class.
But if the player dumps Belkar and picks up a cleric character he's going to be much more vital to the party. Durkon has always been a major contributor, and is always essential. Haley's half of the party has spent months trying to find a cleric. If the Belkar-player swaps to cleric he'd always have a function in the party. Plus they would be far less likely to turn against him or abandon him if he provides necessary healing.
The cleric in question is also a good choice. He's a cleric of loki, so I believe his alignment is CE. Clerics of Loki get the flame domain, which'll let the Belkar-player rain down some destruction. Yeah, the character appears old, but the Belkar-player could have chosen an older age category to increase his wisdom score.
We know that the cleric is less than level 13, because he can't cast ressurection. But if the rest of the party is level 13 then a new character would start out at the beginning of level 12, typically. So that works, too.
But I hope it's not true, I like Belkar and want to keep seeing more of him.