Yes there is a big advantage to keeping your memory consumption low with ruby apps. Ruby's garbage collector is not the best and has to walk all the objects in the process when it GC's. The more memory your process uses the longer and more often GC will happen. This will degrade performance the more memory you use. In fact I've seen really leaky apps spend most of their wall clock time in the garbage collector.

Sure, throwing more ec2 instances at the problem is one solution, but if you care about your apps performance you will try to optimize for smaller memory footprint.