Why should blinds be defended though? Is it because we are already "invested" in the pot when other players aren't (excluding ante games)? Is it connected to being out of position?
It's never because you have already invested anything. That's absolute worst reason to make a call in poker because when you've invested too much in a bad had you're just giving your self a reason to invest more.
The main reason for defending blinds is you are betting getting better pot
odds and you're closing the action. If someone makes a 3.5 BB raise and everyone including the SB folds you have 5BB in the pot and 2.5BB to call. You're getting 2 to 1
pot odds, which are better pot odds than anyone before you got. And because you will be closing the action you don't have to worry about someone 3-bet squeezing you.
So when you think you are 33% or better against villains range you should consider possibly defending. But keep in mind you will be out of position. Against an EP open you should be much tighter than a button open because the EP player will have tighter range than the button, esp. if the button player likes to steal. Always adjust your defense ranges to the player and their table position. You should be defending raises from the button and SB much more than earlier positions.
