I think that it depends on what context it is used in. 98% of the members on these forums are Manchester United fans, there are just the odd few trolls now and again.
I think that if a fan on these boards calls Man Utd "Man U", I don't think that people should get so upset over it. To call members that use it (on accident) "shameful" and "insulting" seems very ridiculous to me.
Don't get me wrong, I'm not wanting to start a fight or trying to make people say it. But I think that unless a member on these boards posts one of those disgusting songs, whatever they want to refer to Manchester United as shouldn't be seen as insulting - as they're not meaning to cause offense.
I actually find it more offensive that people are constantly posting that little rant containing the rival's songs.
I always say "Man Utd" because I don't want someone to copy and paste that rant into a topic and perhaps ruin the thread.
The rant has been posted so many times, yet people still say "Man U" so I think that it's pretty much fighting a losing battle.