Well, "provide" in the sentence "justifying opening android in order to be able to have a consistent platform to provide their services on" has a meaning from the standpoint of the provider. They own the proprietary code of the individual apps (gtalk, mail, voice search, youtube, etc), and in order to be able to _provide_ these specific apps written in this specific code, they needed to open source the platform to encourage other vendors to buy in.
It's actually kind of similar to their stance on the Chrome browser: the web browser itself is open source, and they are providing that in order to enable a consistent platform for their apps (mail, gtalk, youtube, whatever). Just because the browser is open source doesn't mean they need to, or anyone expects them to, open source the code to gmail. If you consider the line between the user and the service to be at the device/UI level, and not at the network level, then this makes sense. They are enabling a better experience by providing functionality that can run more locally to you. This is, however, a huge risk on their part. They know that their software is easier to reverse engineer once it is not running just on their servers -- which, considering the complexity of the gmail android app, makes me wonder why no one has done enough reverse engineering to be able to recreate the UI and still use their backend -- the desktop browser gmail service doesn't require SSL, so it's easy to take a look at the protocol (much easier than trying to network-sniff an SSL session on the android platform itself, if the gmail app uses SSL) and recreate it. And now that they've created and released an app that runs locally they are more tied to a consistent API than they were with just a browser based app, so the rationale of "they'll just change the API and we'll always be playing catch up" against reverse engineering is moot. It's most likely against the gmail TOS though.
Incidentally, I'd like a thirdparty gmail app because I want to access a gmail account that isn't tied to my phone from my phone (like my work account, we use google mail service for company email), however the web based interface is pretty strong. While it's interesting from a UX standpoint to have EVERYTHING tied to one google account, it's not pragmatic and doesn't work with the goal people accessing using/their services.
