That's really my big outstanding question.
2.0.15 is the MarkerClustererPlus version that this builds on. I think it's helpful to track that (the same way that gwt-maps tracks the google maps api version).

But as Joseph has said (below) it's also a bit weird to have one module in the repo have a different version numbering system to the others (even if the logic behind it is consistent)

There's also the issue of whether update to this utility (and others that get added in the future) should be dependant on the latest GWT-maps release. e.g. If someone is happily using maps 3.9, and decides they need clustering, should they need to upgrade to maps 3.10 to get it? 
I think the answer is yes, because in effect we're treating this as a part of gwt-maps - and the story is "the latest gwt-maps release has support for marker clusters" - but there's no technical dependency as such (gwt-mcp only uses MapWidget and Marker, and those have pretty stable interfaces across map releases)