This is an old thread, but I hadn't read this part of the forums until now.
I am against the scaling down to 5 man group sizes. I find that with my older age and 2 little ones that finding time to do big zones is rough as it is. I often have to AFK without warning to prevent my children from damaging themselves/my stuff. I always pick characters which aren't considered "necessary" so that it isn't a big deal if I AFK for a fight or two. I doubt I'll be added to groups to do high end content with the proposed changes because of the above.
I disagree with the idea that people will just add another group. I suspect that if there are 6 people in a clan on, then 5 will zone and 1 will log (and that 1 will probably be me). I say this based on years of experience: even when I have been in clans with 20+ people online, usually there is 1 major group (lead by clan leader and top players) who zones, while the rest solo and wait for a spot. I think this will wind up causing people to get bored waiting for the top group and play something else.
As Reed said, most of the game can be done by 5 man groups already. Dropping the cap from 10 will just result in less interaction between players and result in elitist groups within clans, i.e. the top 5 (not coincidentally, usually the best leaders) are going to kill Cyan, while the rest of you do whatever. Also, groups that typically don't mind adding X newbie/unknown character won't add a spot for them if it is going to damage their ranks/shinies. It would be better to just add a henchman-type system to scale the smaller groups up to 10, than rework all zones based on 5.
Finally, there are a lot of people, including myself, with a decent amount of zone knowledge but that simply don't want to lead. I don't think that scaling down to 5 is going to change that whatsoever.
With all changes, I try to decide whether the change will make me play more or less, and whether I will get more or less enjoyment after the change. I feel like this will result in me playing less and enjoying it less. I solo a lot (easier with children anyway), so I guess this may help with that, but at that point, it becomes a SUD (single-user dungeon) rather than a MUD.