I have tried setting up network mode on the E310s, but have not been able to get it running on either. I have the same version of UHD running on both the host computer and the E310s (UHD_003.008.004-0-unknown), but when I run usrp_e3x0_network_mode on the E310s, it hangs at the line
-- e300 socket accept on port 21761 for I2C
and doesn't seem to do anything else (except for the LEDs on the Rx and Tx ports stay lit, even after quitting the command via ctrl-C). I realize that the E310s were not designed for use via network mode, but everything I have read seems to say that it should work, just not that well.
What you say is true, the network mode is just for diagnostic/testing purposes only, and is currently being deprecated. It will be gone in future FPGA releases, since architectural changes to the FPGA make it impossible.
So, anyway, I think what you're seeing might be correct: The network daemon is now waiting for commands from a host PC; running an UHD of exactly the same version with E310 features enabled should enable you to connect to that. Note that none of the default configurations of UHD seen in the wild or distributed as binaries have those enabled. If you need the network client, you will have to recompile UHD, and make sure to compile/link any software that uses UHD against your specific UHD version.