The "arms free" design of the chest pack makes it best for lots of arm motion or warm weather. I got hooked on them when rowing my Buck's bronco raft out west on steelie rivers, and found they are cooler than vests for those long walks on hot days as well as the arm flailing involved with striper flyfishing. I've got the back compartment accessory that comes right off if you don't need it. My favorite feature is the lack of neck pressure, every vest I own ends up weighing more than any fish I am after by the time I enthusiastically load it up the night before.
For rowing, the bulk is centered on a chest pack to give more room for arms to stroke the oars. They are awesome on float tubes too, particularly those with the flip-out "work-table" feature.
On the downside, they can be pricey. I am waiting for the competition to bring the price down before I buy another one so I can keep each loaded for trout/striper, etc.
With today's nylon mesh vests, heat is not so much of a concern. Some have neoprene neck pads and other neat features. Still, I've come to like chest packs for the above reasons.