But what if it wasn't writeable recently but has been in the past? How do you know that the seed data isn't stale and you are starting up the entropy pool in exactly the same state as last time, and the time before that, and ...
In some security contexts this could be a significant concern.
The seed file will go stale if you deny the system to update it. It's the first source for the entropy pool, but it's not the only source. I really have no idea how large effect the random subsystem suffers as a whole if that source is allowed to go stale.
You usually don't, unless you demand signed kernels and have a secure method of blocking unsigned ones.
But the read-only filesystem issue is something that could happen by accident rather than malicious alteration - for instance some filesystem errors may result in it being mounted RO for safety until the corruption is addressed.
In some security contexts this could be a significant concern.