So I haven't posted here in a while because my time to work on TSM has been pretty low lately. There has been some work getting done though. There was a new version of the Crafting module released just last week for example. Anyways, that's not the topic of this post. The topic on this post is why you should almost always check that "Display lua errors" option (Game Menu -> Interface -> Help).
First a bit of background info. All wow addons are coded in a language called lua (and no it's not an acryonym - it's pronounced "loo-a") and they run completely contained within wow (rather than running side-by-side wow). This is so that for the most part, if there is an error in an addon and it crashes, it doesn't effect the game itself. The game will report the error to the user as a "lua error" (or if you aren't displaying errors it won't display it to the user - but everything else it does, described below, still happens).
That's not all that happens when an error pops up though. Wow will remove the piece of code that caused the error from the addon for the duration of the session (until you reload / re-log). This means that if you got an error when you tried to open some window, if you tried to open the window again, either nothing at all would happen or you'd get another error because the function to open the window is suddenly gone and whatever you did to try to open the window won't be able to find said function causing an error.
If you didn't have lua errors enabled, you'd have no clue why some window is blank or some button didn't do anything or all an addon's features suddenly disappeared and if you went to the author saying "the addon is randomly disappearing", that would be give the author nothing to go on to try and help you fix it. Alternatively, if you went to the author and posted the lua error, that error usually contains everything the author needs to fix the problem (although often a bit of explanation as to what you were doing when you got the error / how to reproduce it is necessary). The error contains info on exactly what went wrong and the specific line of code that caused the error.
Essentially, error messages are the best tool developers can use to fix errors. I'd say 90% of bug reports that don't include lua errors don't end up in any action being taken. So, (and I'm not just speaking for TSM in this post) check to make sure you are displaying lua errors and next time you see one, take 2 minutes to report the error to the addon author. Is 2 minutes of your time too much to ask for the hundreds of hours of the developer's time that went into making the addon?
PS: I would say not displaying lua errors during raids / arenas would be reasonable (don't want something randomly covering any part of your screen during a boss fight).