Again, I wouldn't say "validating transactions".
Nodes validate transactions.
Mining:
1. offer an anti-spam protection service
2. creates a way to choose which one is the block to be considered valid without relying on any third party.
Discussion
point 1 is exactly why it's right what Ocean is doing
nodes can help limit spam also, though the effect is very marginal, one never knows what path a new tx propagates versus when a block is assembled and target hash is found from it