Having a large amount of accumulated fuel in the combustion chamber when combustion commences is a bad thing, for a number of reasons:
I) It increases emissions, particularly NOx. Generally speaking, NOx is created when an oxygen/nitrogen mixture is subjected to high temperatures and pressures. At the start of combustion, the combustion chamber on a diesel engine is filled with air, which is a mixture of oxygen and nitrogen. It is under high pressure. It is fairly hot. If a large amount of accumulated fuel suddenly ignites, creating a very hot flame front, the process will probably create a large amount of NOx as well.
II) It makes the engine noisy. When the fuel does combust, the more there is of it, the bigger the bang. Technically speaking, a large amount of accumulated fuel quickly combusting will result in a rapid increase in cylinder pressure, something which humans perceive as the characteristic diesel knock sound.
III) It provides localized heating of the combustion chamber. If a small amount of fuel is accumulated when combustion starts, it will more easily mix with the air in the combustion chamber to minimize combustion chamber hot spots. When a large amount of accumulated fuel is present and burns quickly, there is less time to mix with the air and localized heating is more prevalent.