I've been thinking more on this. An issue is that there are at least some people who want XP support (for whatever reason) but there aren't people who feel particularly motivated to support it.
So, as smj389 says, separating Windows XP target from the Windows 7+ target would better reflect this. It may also provide a target to rally round if there are people willing to come forward to offer XP support.
An alternative to having a separate XP target would be to somehow allow users of Rust to set the Windows "platform toolset" / "sdk version" for their project, as they can in a Visual Studio C++ project (this would actually be useful regardless). But note that even in Visual Studio, XP is treated as a special case (and it's currently marked as deprecated). I think Windows changed a lot under the hood from XP to Vista and this is reflected in the dev facing APIs, so considering them different platforms is warranted.
In summary, if the goal is to better support XP then having an XP target helps. On the other hand if XP support is (as now) more or less left to rot then having an XP target will at least keep the rot from affecting or holding back the Win7+ target. If official XP support is to be dropped, then separating it into its own target could be a prelude to that.