I have been wrestling with this one for years. I am a sucker for baggage, especially for small pouches to provide a place for everything and everything in its place. That's why I find this an interesting thread.
For some time, I used a 7 litre Macpac bumbag which I modified as a chest carry for small, ready-to-hand items and to fulfil the functions of a response pack. However, I found that it got in the way when trying to get through small gaps in undergrowth or rocks. More important, it restricted my view of my feet and where I needed to put to put them when descending steep mountain paths. I have now retired it.
A chest pack seemed like a good idea at first to avoid taking off a heavy pack to get at, say, sunglasses but I found a chest pack was not the answer for me. I have since reverted to my older practice of attaching a pouch and a water bottle holster (don't like bladders for a variety of reasons) on either side of pack straps or waist belt. Following the example of my younger son, I keep a small foldable day sack containing waterproofs and emergency items in the top of my main pack. This goes with me on side trips together with the water bottle and small pouch. I notice that Joe O'Leary also advocates this practice.