argh, I had a nice long post and like lurker, lost it when the forums reset.
I'll try it again:
I played a match in Dota once where every player was given the same hero. As it turned out, we all got Magina. Magina excels at killing casters. His skills include mana burn (every hit takes mana away from the target, each point of mana taken does bonus damage), blink (short teleport with a 5 second cooldown when maxed), spell shield (up to 40% spell damage reduction when maxed), and his ult is a long range nuke that does damage equal to the amount of mana depleted.
So basically Magina will initiate a fight with a caster (int hero) either by walking up to him, ambushing using fog of war, or blink in. As he attacks he's doing regular damage plus mana burn, and in the process, draining the casters mana. Perhaps the caster will hit him with a nuke or two, which he'll shrug off due to high spell resistance. Then they may try to run. Using a 5 sec cd blink, and having depleted his opponents mana, his ult will finish them off. He's effective against other types of heroes, but is best against casters (which is why his title is anti-mage).
When all of us were Magina, there were no casters to attack. It became a game of chicken where one magina would attack another and blink out just before his mana was depleted too low to cast blink. Items become the only differential, although most went with blade mail (20% damage returned to attacker). It was a crazy match.
Was it fun? yes. Did it get old? yes. Would I do it again? no. Did it feel like a real match? no.
Now I'm not saying that you'll likely see everyone with the same demi in demigod. Hopefully all of the demi's will be balanced out enough that everyone won't want the same demigod, or the same 2 or 3 demigods. But my point is, when you have multiples of the same demigod it warps the playing experience.
Yes RTS's allow for players to pick the same race. Normally there are only 2, 3 or 4 races to select from so its an obvious neccessity. Also, the races can be very unique from each other depending on what units you decide to build and what build/tech strategy you employ. With demigod, the only real distinction will be items and a slight difference in the order of skills you choose to level.
I like to feel unique on the battle field. The only game i've played like Demigod is Dota. It just wouldn't feel right if there were 3 or 4 of the same heroes in dota. You can't distinguish player 1 from player 2 very well since they have the same hero. I'd hate to have 3 lina's stacking their ults on a couple of heroes. I'd hate to have 5 zeuses with refreshers tko'ing the other team every 60 seconds. I'd hate to have 20 meepo's running around on a team. Do you see the point? Each hero in Dota has been balanced so that it will be the only one on the field. Sure Demigod could go in a different direction, and balance it appropriatly, but I can't wrap my head around it.
Just my opinion of course. Your entitled to your own.
PS "Lurkher, you re right when you say "In fact, I'm convinced, multiple picking hero will be available, so you can say what you want, this is useless."" there's no basis from this besides your own gut feeling.