Long time reader, first time poster (now I feel like I'm on radio call in show).
I agree that app developer can't really remain neutral as far as templating engines are concerned. However, toolkits and frameworks, like CherryPy and TurboGears, that want to intentionally remain neutral (or at least flexible) as far as templating languages go really stand to benefit from this plugin system. Plus, since there are so many "normal" templating languages, it is handy to have a common means of getting at their functionality. Since we've got a bazillion template languages in Python, we might as well put them to good use ;-)