Over time it will get a bit smelly, and will probably get a build up of smegma. 

Nothing too harmful at all really.  The chances are that when you urinate you are cleaning under the foreskin a bit anyway.  If you simply cannot clean it because you are, for example 000s of miles away from water hiking through the forest or something, then just by urinating and pinching the end of the foreskin lightly will cause more urine to flow around the glans inside the foreskin and have a cleaning effect. 

Of course, if you are not 000s of mile from water, ideally you will be showering or bathing at least daily and cleaning under your foreskin, assuming you can retract it properly.  If not, see the instruction on stretching at the top of the mens health board here. 

When the guy or others around him are starting to smell then you know bacteria is starting to build up and it can cause problems for the guy and or the mate.  Some women have the same problem(in a much smaller way) depending on how they are built.  Even an inward belly button will get smelly and sometimes irritated if not kept clean and dry.  Folds of skin on anyone(the over weight or women with hanging breasts) can be a potential for bacteria to grow and cause problems...even dogs with many folds need special attention like bulldogs face and others.  The best thing is to keep it clean.