Crema is wonderful stuff - a soft, rich foam holding all those wonderful aromatics. It tells us much about the extraction and the coffee, and its presence is no doubt required and should be sought.
But you can have too much of a good thing. This has become more pronounced with the appearance of crotchless pfs - where you get a very very thick crema.
Has anyone ever got a teaspoon and tasted a little of the foam on its own? Its often not terribly pleasant. Mixed with a little of the liquid below its superb but when you have nearly an inch of crema its all foam no fun.
The other thing we seem to be after is persistance of crema. Why? Surely we should be drinking the espresso as soon as possible - and arabica will easily provide a crema that lasts long enough. The customer must wait for the espresso - not the espresso for the customer.
Please shoot me down, disagree with me. I don't believe that crema is the be all and end all of an espresso, just one important part of it.