It's true that a datacenter network is slower than RAM, of course, but if you're already dealing with internet latencies, an extra roundtrip within the data center is hard to even measure - see "Latency numbers every programmer should know", http://www.eecs.berkeley.edu/~rcs/research/interactive_laten....
(I really don't like this fact, aesthetically, but it's usually good business to acknowledge it.)
Sure, for most people it isn't an issue. But sometimes it is. E.g. if you're hosting some kind of product that collects or inserts data into customer's websites.
If you are really aggressive about performance you'll have data centers within 100-200ms of all your customers. If you do this, and you buy into the microservices, it doesn't take many inter-service calls to exceed the network latency. On the TechEmpower benchmarks most frameworks struggle to return a static bit of JSON in <50ms. Now imagine 10 services communicating to fulfill a request...
(I really don't like this fact, aesthetically, but it's usually good business to acknowledge it.)