A big benefit of how opinionated frameworks preempt debates about marginal issues is that it's an externally imposed decision, instead of one team member triumphing over the others.
It really helps team cohesion for DHH to tell you that you're using X instead of Y as opposed to the most domineering devs getting their way every time. In my mind this even compensates for some suboptimal choices Rails may make.
When working with a framework that doesn't make those decisions for you, you end up with two big downsides - you waste time and energy arguing over which of these low-value choices you want to take, and new developers take longer to come up to speed with your particular architecture, since they need to learn each of the choices and how they are integrated, whereas in Rails it's obvious where everything is and how it's configured if you're familiar with the framework.
Bloat is also over-emphasized, particularly for the backend. You'll load the gems in memory, sure, but they're a fairly small footprint on their own and it's not like Rails is calling the parts of the framework it's not using (save maybe middleware, although most of that is useful for most web applications).