At least three obvious reasons might be: (1) Raw scale might mean you just plain can't fit everything on a single t2.2xlarge. (2) Different services (containe

At least three obvious reasons might be: (1) Raw scale might mean you just plain can't fit everything on a single t2.2xlarge. (2) Different services (containers) might have different performance profiles, so you may want a few different types of machines around. (3) You probably still want N+2 redundancy even within your single AZ, so this scheme should at least be upgraded to three t2.2xlarge boxes. ;)