The layering - defence in depth - is very much a security consideration. Especially if you're building a pure request/response/sync system you need that. Or you decouple with a queue for mutations and avoid a lot of issues.
That may be in terms of managing general security, especially with regards to the attack surface of the solution, but here we are talking about DDoS, which is mostly a separate topic and handled on the network level (for volumetric attacks) and load-balancer level (for non-volumetric attacks) or a combination of both.
