Do you periodically retest them to ensure the ones you are encouraging people to use continue to work as intended? Or are the templates not expected to work the same in all the email packages you test agains in the Design test suite anyway?
Impact for example is pretty broken in Office 2007 & 2010.
There will be some variation in how the templates look, to deal with different capabilities, but we do also revisit the code to improve it from time to time.
If you'd like to drop us a line to support about the problems you are seeing we can check it out too.