> Perhaps you don't want everything eager loaded, but you want an exception to be raised if you try to access an associated record that hasn't been preloaded.

Yes, exactly.

> When I need to load/use them along with many parent records, it seems pretty obvious that I'll want to include those associations (eagerly loaded) in my AR query to avoid N+1 queries as you mentioned. Then again, maybe I've just spent too long taking those assumptions for granted where newer devs might not.

You know that and I do too, but legacy Rails apps tend to be full of N+1 queries in my experience, and it's a major cause of slowdowns.