I do agree with you that unless RLC becomes BIG, miners will be a very small part of it. But, there is always that possibility. And if/when it did become big, it would be nice to know that there is built-in protection against DOS attacks (or accidental downtime, maintenance downtime, etc). The advantage of being able to transact without connecting to a central server could prove to be very important in some cases.
What built in protection? The mining doesn't replace the confirmation by central authority. It is simply supplemental. So if tomorrow RLC has 9TH of hashing power the central server could be DDOS and the mining would serve no purpose.
Transaction signing with a central server is going to have some disadvantages, and I think mining could alleviate those disadvantages if it is ever used enough.
How? The amount of mining is completely immaterial. 1 million miners, 1 miner, 0 miners. The whole point of mining is to create a distributed consensus. If one authority has an override then there is no purpose except to waste a lot of resources pretending the network is distributed.
Did you miss the part where he said the central signing would be removed when there were enough miners to take over?