I agree with you, also i'm not sure if users would be willing to "fix" a template which will never be used alone. Even if it works like that, changing tablestop to a different valid syntax would break the including pages. There, you can't forbid saving. Moreover, the editor is not going to notice that the change to template:tablestop made blinking red warnings on several different articles.
Inline error messages which are generated *after* templates are included should work. Anyone viewing Template:Tablestop would see an error (and can ignore it), but pages correctly transcluding the template would be valid and would not have any error messages.