How do you prevent an attacker from getting the seed? This would just create a possible attack vector that could be used to effectively disable address space randomization.
e.g. only root can get access to the seed. Then the attacker would already need to have root, so then you're in huge trouble anyways.
And yes, it may be weaker than a full-random solution. But a pseudorandom system that gets accepted is more secure than a full-random system that doesn't get accepted.