Could there actually be some benefit to having a bit of padding while it is still easy to do so? I can honestly see both sides of this argument. I can see how being very lean and muscular would exact such a high metabolic cost that a transition into a scenario where food is no longer easy to come by could be problematic.
It probably depends on your defintion of "fit" - If you are looking to compete with trained Marines 22 years of age, then yeah, you probably need to be lean.
I amnot sure "fit" needs to be that. As long as you are fit enough to do the work you need to do, I am sure the slower metabolism helps. After the work I did last summer, I am pretty sure my body wants to maintain pudge even as I get stronger. The reason that its so dang hard for humans to maintain that perfect body is because Nature doesn't want us to. Mother Nature wants to store some fat - or at least she sure does on me. Fat doesn't mean you are not fit. You need your cardio-vascular system to be up to speed, and your muscles need to be strong enough to do what is required. Being fit doesn't mean looking like a Greek God - that is an artifact of human vanity ( of whose measure I apparently got too little) and a desire to keep your metabolic rate the same as it was when you were 18. Its far more important to be healthy, and being so probably does not mean pushing your body to extremes metabolic rates it doesn't do naturally in order to get rid of fat it wants to keep at age 40.
If you don't know what you are doing, moderation seems to always be the wisest choice.
