First of all your logic is somewhat flawed. Your logic only holds true to someone teasing a PK line. I for one would never tease a PK line, because like you said it won't give me much of an advantage.
However, teasing a +9 line, as was the case with the browns last night was very smart. With a 7 point tease I am now getting a 16 point handicap, which is basicly 3 scores!
My NFL teasers usually win 90 percent of the time.
You do have to know a bit about the game to make it profitable. You can't tease without knowledge of stats and trends either.
If you know what you're doing teasers can be a great addition to your betting arsenal.