I agree with you on the notion of fragmentation, albeit not on Google Play.

A far greater concern, in my opinion, is the fragmentation of the market due to anticompetitive practices like vendor lock-in. This would be things like Amazon vs. ePub and the proprietary versions of ePub (i.e. the different DRM). Kobo is okay on this front at the moment since they provide ePubs with Adobe's DRM, but who knows if this will continue in the future.

With respect to Google Play, well, this isn't exactly the iOS walled garden. Android vendors do have the option to use different marketplaces, including their own website), so it is actually the vendors making the decisions here. I don't always agree with those decisions, such as Amazon making it difficult to access the Kindle app outside of Google Play and their own marketplace, but it is Amazon who will potentially suffer from that decision.

Alas, it is too easy to treat a company's actions in both respects cynically because there is too much vendor lock-in going on. As an example: did Kobo originally neglect the Google marketplace because it didn't meet the requirements, or because they didn't want people to buy their device to use the Amazon, B&N, or Sony book stores?