I'm trying to figure out how much of a "sin" it should be considered. I mean compare it to offering a -7 -110/+7 -110 lineset, but you have it set up to not take bets at -7, only at -7.5, you offer -7.5 and if the player wants it, you take it.
The "the player doesn't have to take anything he doesn't want" argument still holds there - you don't have to take -7.5 if you don't want to.
So you are a book that isn't offering 20 cent lines, but again, what's wrong with that? If someone wants to set up a 40 cent book, that's fine.
So then, if neither of those things is wrong, then why wouldn't a book just offer -7.5 -110/ +7 -110 lines? Because even the squarest of squares would never play there and think the book was ridiculously bad. So THAT'S why it's bad if a book does that. Because there is huge, huge, huge value in "advertising" or "presenting" yourself as a -7 -110/ +7 -110 book.
So if secretly offering -7.5/+7 lines while publicly trying to make it seem like you are offering -7/+7 lines is so bad, as durito is saying 5dimes does and as other books have done, then how is that different from dealing dual lines? Because the line sets you are dealing still all have the same juice? Then why not just come out and say you are dealing dual lines, just like a -7.5/+7 book could just come out and say that they are dealing those lines?