Wha . . . but they could fight before . . .

[adrich]

I think you're basically making too much of it. Games don't always make sense, especially when you start comparing gameplay to story.

Liukan is able to fight in Suikoden, but you also have to put him in your party at one point and there's no convoy or support feature. Suikoden III does have a support character feature which means that some characters can become more useful outside of battle. When some aspects of the game change other aspects are also expected to change in response, whether or not they make sense for continuity is another matter.