This was one piece of equipment that I thought was a waste of money. I had one in my hands at a shop that took hours to travel to but decided I didnt need it.
Well, after shaving the head, porting, advancing the timing and fitting in a bigger carb, I have improved power from increased compression and freer revving up high. The engine runs noticeably much hotter. Without water cooling, I then realized the purpose of an oil cooler.
A friend of mine has got one of those 140cc's from XY (high comp piston, IRK, etc..etc...). Damn, does his engine run really hot. The engine came with an oil cooler and it sure was for a purpose.
I'd say for a standard engine, there may be no immediate need although there would be no harm in having one. For modified engines, its practically a must.