First of all, I don't see the need for a seed. Since the seed has to be stored with the private key anyway, you might as well regard it as part of the private key.
You've missed a whole use case here:
Say I want to run a webserver that accepts paymets. It needs to be able to generate addresses, but if it gets hacked, I don't want the hacker to be able to spend any of the incoming money.
By splitting the master private key and the seed used to generate the addresses, a RX only wallet can generate unlimited new addresses without having the ability to spend or any required communication with a separate secure wallet that can spend. An attacker who stole the data on the webserver could only deanonymize payments.
Thats why I proposed it with a separate seed.
Perhaps not important for all uses, but pretty useful for this ecommerce one.