One thing to think about is the inherent flexibility of titanium and its effect on paint.
Many years ago, a customer at a shop I worked for brought in a painted titanium bike that was showing cracks at the driveside bottom bracket/chainstay weld. In order to determine if it was just the paint cracking as a result of frame flex or if it was actually a frame crack, he ended up having to strip the paint in that area. Fortunately, it turned out to be the paint, but stripping it ruined the look of the bike.
I've always assumed that this was the reason ti frames are usually unpainted.