In this oldie but goodie, Hampus Sethfors digs into why disabled buttons are troubling for usability reasons and he details one example where this was pretty annoying for him. The same has happened to me recently where I clicked a button that looked like a secondary button and... nothing happened.
Here’s another reason why disabled buttons are bad though:
Disabled buttons usually have call to action words on them, like “Send”, “Order” or “Add friend”. Because of that, they often match what users want to do. So people will try to click them.
What’s the alternative? Throw errors? Highlight the precise field that the user needs to update? All those are certainly very helpful to show what the problem is and how to fix it. To play devil’s advocate for a second though, Daniel Koster wrote about how disabled buttons don’t have to suck which is sort of interesting.
But whatever the case, let's just not get into the topic of disabled links.