whats wrong with a taint?
 

"Interface action failed because of an AddOn"

why is it frowned upon? any negative effects other than being annoying and looking ugly?

In most cases this doesn't really matter, however the message is there because it's entirely possible for the taint to break functionality in either the addon or the default UI itself.

If you see that message then either functionality of the default UI has been broken or an addon is trying to do something that Blizzard doesn't allow addons to do.

Regular taint is (usually) fine. Whenever an addon touches a secure part of Blizzard's code (even just a variable), it "taints" it - meaning it's no longer pure. But the "Interface action blocked" messages mean that the taint is severe enough to actually prevent the default UI from doing something or, like Arrowmaster said, the addon is trying to do something that it is not allowed to do. These messages usually pop up in combat, when the default UI locks down and is more restrictive.

Yeah, it doesn't always have a real effect. For example, if you try to resize/scale the world map while in combat and still have the 'blob' frame (the blue highlight for quest objectives) showing, you get one of those taints. But nothing actually breaks.

Taint is dangerous because it spreads through execution path, so even something that is seen as a innocent taint might have adverse and weird effect on the overall UI at a later time, or when something specific occurs. This leads to confusion about why and how some weird bug or unresponsive UI occurs when it's in fact a taint.

So overall, I would suggest being very careful and conscious about taint. For instance, a lot of people use the UIFrameFade/UIFrameFlash API as if tainting them had no consequence, where in fact it breaks the default UI when someone uses the internal voice chat.