I think the concept of reciprocity is mainly there so it doesn't feel like the friendship is one-sided. Different people have different skills/talents. I don't cook, but my best friend does--so she might host me at her place, but I'll take her out for a nice meal. And, maybe I spend a couple hours helping a friend out with a project or issue one weekend, but she gives me rides on a regular basis because we live close and she has a car...
Also, some people *love* hosting, can afford it, and have a better space. I think there are other ways for friends who are uncomfortable hosting, can't afford it and have no space to make it clear they aren't just mooching, but it doesn't really make sense for both friends to host equally often, in the situation where one is genuinely much better suited for it.