I wont disagree with anything in this thread. Like Supremor said there is Proís and Cons to both sides of this argument and I doubt that they will get worked out here.
As far as the loss of dexterity goes, I started conditioning my right hand when I was about 16 yrs old. I am currently 34 (I stopped the conditioning around the time I turned 21) and I build large-scale networks for a living. I spend hours on a keyboard and I only have problems with my left hand. After big jobs the left hand hurt so bad at times it wakes me up at night, my right hand has no issues at all. The old guy that I used to work out with - he taught me most of my stuff - had hands like clubs, he was an amazing artist. I guess what I am saying is I know several people with very hard hands and donít know any of them that have issues because of it. I am not saying this canít or wonít cause issues long term, but my personal experience does not support it (the old guy was 70+).
On a side note, when students ask about conditioning, I typically try to talk them out of it.
The irony of this thread for me is that I am currently nursing a messed up right hand. I have been out for some time (about 14 years) and decided to see if I could still break like I used to. I stacked up 8 patio slabs to see how many I could break. I pulled the punch and I think I actually broke something and I am not talking about the concrete.
Big breaks are cool and all but I actually prefer the small quantity technique breaks, such as placing your hand on the board/block and breaking it just by changing stance or breaking SLOW (think Tai Chi slow). I can teach anybody to power through cement, but these are the breaks that require years to figure out. Conditioning for these breaks are not necessary.