I guess I should start by saying that my nutrition background comes from lifting weights, not from cycling or endurance sports. Thats why I asked if I might be missing something. However, that being said I do know that muscles don't recover overnight. They just don't. Even if you're not sore the next day or even the day after that, your muscles are still recovering. Thats why people don't work out the same muscle two days in a row at the gym, unless of course they're doping. Obviously the goal of cycling or running is quite different from body building or power lifting but that doesn't mean the same rules don't apply. Every day when you go out and ride you are breaking down muscle tissue. After you're done riding your body needs post workout nutrition to provide your muscles with the building blocks to repair themselves, protein. Now here's where people start to disagree. Casein (milk) protein is slower to digest but it gives you a sustained release over time. Whey (egg) protein on the other hand digests much faster and gets to your muscles much faster but it doesn't have that same sustained release that casein has. One of the big debates in nutrition is whether or not casein digests fast enough to provide the optimum amount of protein post workout. Unfortunately there's no way of telling, atleast not yet. As far as I know every study on this has shown whey to be superior immediately after working out while casein is better later in the day or before you go to sleep. To be on the safe side many people will drink whey protein immediately after a workout to ensure that the initial demand for protein is met and then drink casein later to provide a sustained source of protein throughout the day. Now some of you might be thinking, why not just take both at the same time? Wouldn't that be convenient. The problem is your body will digest the slower protein first. I'm not a doctor so I don't know why its this way and I'm too lazy to attach links but thats just the way it is.
As far as the high GI carbs in chocolate milk causing you're insulin levels to spike, thats a nutritional fact. Sucrose (white sugar) is artificially processed and like other processed carbs it is converted into energy almost immediately, i.e. sugar high. What nutritionists have learned over the years is that insulin plays an important role in muscle recovery and it is optimum to eat or drink something that allows for sustained release of slow digesting carbs which in turn creates sustained release of insulin. For this reason you should consume natural carbs like "old fashioned" oatmeal, brown rice, etc. Another option, since nobody wants a big bowl of oatmeal after a long ride, is a sports drink like Gatorade or fruit juice which contain unprocessed sugars.