Doesn't seem fair that somebody can come along and offer a service for free/no ads in order to get people to switch over from a provider that charges/has ads, only to eventually switch over and do the same.
Considering that they do not lock you in (they allow to export your content), I see nothing at all unfair about it. If you have your own URL that you point to your blog, switching should be pretty seamless as far as your readers are concerned.