I know that it is generally "taboo" to put more than one lead in a plated through hole, but I would like to better understand why.  I believe that it is because:

1) Rework:  You need to be able to remove and replace any component without impacting another.

2) Flux entrapment and solder defects:  The oversized hole required will result in increased solder defects and could also lead to flux entrapment.

Are these reasons correct and are there any other reasons?

Are there any classifications of PCB which allow for this practice?  

Anyone out there with real experience (positive or negative)?

Thanks,

Lang