It's not an obligation, but get an equilibrate hashrate between all pools, is better for the reputation of the coin.
Currently hash.so has more than 51% of the hashrate, so the mass think than this coin could be hacked, but as said hash.so's admin with a big pool, lot of altcoins could be hacked.
I have had >51% almost all the time since the launch, I did shut down registrations for a while. I re-opened when the pool was around 30-40%, and well it climbed back up pretty quickly.
In reality anyone who is running a large scrypt pool could easily point all the miners to a small 500Mh/s coin and 51% the hell out of it. It doesn't matter what they are mining at the moment. They would only need to switch everyone for a few minutes which the miners would not even notice. (disclaimer: I don't know how 51% attack really works, so I might not be correct
)
Still waiting for my launch pool bounty