The main motivation is that it simplifies development of experiments in case one needs version control or complete disconnection from SVN trunk for some period. Thanks to git-svn, it is possible to push changes back to the trunk.
I outlined the process of maintaining GDAL trunk using Git in the Wiki article Using Git To Maintain GDAL Workflow.
The synchronization is not a time consuming process at all but even that I hope to make it automatic process in near future. First, I have to ship my little personal server to a remote data centre and I’m hoping to do it next week. Lucky bastard going to bask in the warm sun of Portugal :-)