I have had quite a few uncles (some of which more correctly would be called uncles-in-law). Two smoked themselves to death, one died from something like MS or ALS, one went to China to help out with some construction project and was run over by a truck, one is mentally ill (some sort of schizophrenia), and three are mostly unexciting middle-aged (or slightly older) men. None of these would qualify as crazy in the funny way.
Some dictionaries (including the E2 write-up) will tell you that uncle may also be used for a male friend of the parents. That is where I found my crazy uncle. Amongst other things he had a motorbike, a big supply of candy in his basement, multiple TV-sets (for some years we had none in my home, so that was a big thing) and guns. In Sweden only registered hunters and sportsmen have guns, (rifles mostly), so that was fairly exciting for a seven-year-old boy. Once he let me hold his Magnum. My eyes tingled like it was Christmas and I wanted to run to my parents and show them this magnificent piece. My crazy uncle advised against this and I understood completely, daddy wouldn't understand.
These days I'm an uncle to many. Being a few years younger than my three brothers I can play the part of crazy uncle and still act approximately my age, which is nice. You see, my brothers, nice people as they are, could be considered quite boring and from my own childhood I remember the wonderfulness of a grownup that still knows how to have fun. So I've taken it upon me to supply my nephews and nieces with a crazy uncle.
Who else would try and teach them differential calculus before they can handle addition with numbers greater than 10? If you haven't already I can really recommend trying to explain some advanced mathematics or physics (like the quantum kind) to a seven-year-old child. It's funny and a brain exercise at the same time! And kids really love it when you talk to them like they're smart (which, of course, they are). When the safety-considering parents ban anything funny, who will teach the kids the use of a super soaker? Who will push their soapbox racers around when their fathers' backs are aching? And at mind-numbing speeds even? Who will have LEGO and radio controlled cars in their book-shelf for them to play with? And the rest of the shelf filled with comics?
If I never have kids of my own, being the crazy uncle, with all of the fun and none of the responsibilities, will be a quite nice substitute.