Think of it like this. Your iOS device assumes that if it's on the same network, and it has a valid/current DHCP lease, it will use that address while it goes about the business of negotiating a proper DHCP address. The assumption is made outside the scope of negotiating an actual DHCP address.
A violation of the protocol would involve tricking the DHCP server or sending false information to the DHCP server. Neither happens here.
It's up to the network owner to decide whether or not this is acceptable. With a properly functioning network equipment, you should never encounter a failure of this assumption. If you have an unhealthy network, the assumption may fail more frequently, but then again, you're just compounding an existing issue that should probably be resolved.
In the end, we're talking about saving 10s every time you wake your device up. Ten seconds. That's significant. I use my iPad/iPhone very casually. I don't think about "grouping" my usage, because it works instantly when I pick it up. My network works properly, so I'm happy it's making these assumptions.