When does an API Gateway with Request Aggregation become too much, like, overkill?

Everyone talks about API Gateways combining backend responses for clients. It sounds great, especially for mobile, because it cuts down on round trips. But when does that actually become more trouble than it's worth? Are there times when all that extra work for an aggregation layer in the gateway just isn't worth it? Like, if your backend services pretty much give clients what they need already, or if the combining logic is super simple, should you really put that logic in the gateway? Or should clients just hit the different services themselves?