Today it's not so much a problem, as all IRC clients easily can connect to as many servers/network as necessary - but we're dropping the "relay" part and "network" part if everyone is supposed to run their own standalone server or their own small network.

Back in 1996 it was a also a huge practical problem for the end-users as (almost?) all client software assumed there should be one and only one IRC network, after the split one had to choose what network to connect to.  Being connected to both was of course physically possible, but impractical.

In a perfect network topology, there shouldn't be such a great difference between clients and servers - everyone should be able to connect to the network and relaying messages.  As said, with IRC every server administrator efficiently was a super user.  This meant it was non-trivial to add new servers - every new server and server admin would need to be approved and vetted by the community.  That's the reason why they had to throw out the Eris server - it was too liberal on whom was allowed to connect as a server.