Let me start by first saying, I fully appreciate the need and reasons behind having the XBox Live branding on the Windows Phone be a marketplace tool to help identify AAA phone games to the customers. It does a lot for the platform to have some part of the catalog include titles that have (generally) higher production values, QA and all the rest that comes with the responsibilities of an XBL contract. Limiting access to leaderboards, friends, avatars and other XBox Live features is, for the most part, a reasonable thing that allows Microsoft to ensure the quality of games using these features meets their standards and gives a little more value to the developers who make the effort to be part of XBox Live.
There is one feature however that really, really, needs to be made available to all Windows Phone game developers: the gamer tag. Providing access to this will allow developers who who have any kind of online component to their games have some kind of reasonable expectation that we won't be exposing our gamers to profane or otherwise unacceptable names. It also makes the task of porting games between XBLIG and WP7 that much easier, because XBLIG games have access to the gamertag (despite not being XBox Live products).
Right now, smaller developers have no choice but to accept user generated names, feed them through banned word filters and implement some kind of reactionary cleanup system for when bad names do sneak through. Unless we make users do a registration process or access unique phone IDs (which has its own set of issues), we are unable to uniquely identify users. This is really not doing anyone any good, and it makes all non-XBox Live WP7 multiplayer games suffer as a result. The obvious problems are multiplayer experiences that have parental control issues. An even more serious problem for the developer is that we have to spend an inordinate amount of time dealing with the fact that there is an API that could give us a safe player name, but simply chooses not to so we wind up spending a lot of time on something that could otherwise be used to make a higher quality game.
I can only hope that the right people at Microsoft will change their mind about letting WP7 apps access gamertags because it would improve the quality of multiplayer WP7 games for the entire platform. This is, presumably, a simple switch that could be done in time for Mango. I'm not holding my breath for this change but it would be great if it happened.