Plus side
1-generates revenue
2-improves the world - usually multiple team owners are better coaches
3-coaches enjoy it
Minus Side
1-implicit collusion is in effect as how can one compete against oneself?, esp in same division, although divisions overlap more than they used to
2-advantage in giving coaches a 'feel' for limits in recruiting, esp in same division
3-some have used it to cheat, IMO a very small %
General
1-Problem is worse now in that recruiting between divisions is more grey, and I for one look around the entire country for 'bargains' much more than I used to
2-on the implicit collusion issue, multiple id's is not the only example, another might be a father and son, brothers, friends who got each other involved, friends from the game you sitemail or who you have helped or have helped you, conference mates, etc
3-Overall some may disagree, but I contend that any information is helpful, getting to see another teams data, is helpful, at least to me it has been
On the issue of full disclosure, I still play Ryan75 in world 7 (I think), recently when my OR account went from UCLA to UNC, I left a very good (OK great) barton team and shifted to EOregon with Ryan, so I was not close geographically. When I was at UCLA, I once found a recruit when bargain hunting at UCLA on the last day, a 480 center who was high at everything, I felt so bad that he would have been a great recruit for my Barton team, that I felt compelled to take him for my UCLA team, the point is, even the most well intentioned coaches, will, even if by mistake, probably run into some interference in the game between the 2 ID's. I took the ryan account team on to get rid of rewards points, thinking they might go away at some point, I am almost out - switching to eOregon from barton should guarantee running out, and I think that will be it for double teams in worlds for me.
If I were in charge, I would limit us, to one ID, one world, simply my opinion & not meant as a criticism, based on what I know and how I feel about the game.