Pre-ordering a physical copy of a game has never made sense to me other than if there is an actual, worthwhile piece of content that you get if you do so (which IMO is rare to never). Unless it's like a crazy rare game (again, do those even happen anymore?) why are you going into a retail store and giving them your money before they have the product? It's just a waste of time to me, not some kind of moral faux pas.
Pre-purchasing a digital game? Depending on the game I have no issue with this especially with preloading becoming more prevalent. It's not THAT difficult nowadays to know you're going to like a game pre-release. What you have to be careful of is the multiplayer-focused games now that have had less than stellar track records the last few years.