I design and sell the odd mod and my take is I like to personalise a game but it needs to be removable so the game can be returned to original, and I try to integrate into a game and where possible capture a feature within the art / game. I try to go for the "it should of been factory" look - but this is a challenge and people will always have an opinion and disagree.
End of the day add mods because you like the look or function, it is a personal taste thing and some people like to have some unique.
I designed the snackbar mod for CFTBL for me, and then lots of people showed interest but I did it because I wanted it for my game to add a 3d look for that area - the interest just followed. I am now designing a Milano Throttle Shooter because I want to try the idea out - I am sure it won't be cheap but it is for me and if people want one then I will decide that then if I make more.
As for buying games with mods, depending on the mod you could always determine the re-sell of just that mod and take that off the game - one way to offset the price.
