I think the difference is that Homeseer doesn't make and sell their own equipment. X10.com wants you to buy their modules, and they have no incentive to add in modules from other manufacturers.
As a user community, there is nothing stopping us from creating a "compatibility chart," which would help users who have purchased modules from other manufacturers figure out how best to configure them (for example, I have found that my Insteon ToggleLinc relays work best when defined using the "Florescent Decorator Style Wall Switch" module in AHP.
However, I don't think X10.com is going to invest the time to research that and give us that information. It isn't worth it for them.
It would be nice if they allowed us to add in custom-defined modules, but there is still a cost involved with writing that into the software, and they are less likely to spend the money on it, since they won't see a return on that investment. They are better off using the development resources to write more plugins, which they sell for profit.