Short answer: It's only bad to someone who is unable or unwilling to do it.
Long Answer: Because there's always someone, somewhere who loathes the idea of *gasp* planning a build out or *double gasp* NOT bumping up a useless skill even if your character might have been exposed to it in his/her background. IMXP my group has two players who are of this mindset (one who believes it's a form of metagaming to build a character based around the theme of the campaign. e.g. if you know the campaign is going to heavily feature undead, it's metagaming to make a cleric optimized at turning as opposed to a cleric with some turning ability but not everything focused on it), and I routinely argue with them over my characters (my characters have a concept which I then build and expand upon to make them good at what they do) because my characters are normally better than theirs and they seem to think I am min/maxing because of it.
In short, it's viewed as bad because people aren't comfortable with the idea and think that every little detail should roll out of the way the campaign progresses (e.g. if your character had difficulty in a social situation, you should up his Diplomacy score), and seem to think it's a mortal sin to have a 20-level build all planned out beforehand because it doesn't allow for any flexibility to change during a campaign. These are the same people who would, IMO, change the rules if they could so that the DM should be allowed to deny character progression if the choices don't reflect what "[your] character would do".