It is NOT cruel to clip the wings unless they are kept in a flight aviary.
If you are keeping it in an outdoor aviary where they have lots of room to fly about, then do not clip the wings. However if you are keeping it indoors in a cage, DO clip the wings. It will only take 10 seconds and an open window for your bird to disappear and there is such a slim chance you will ever see your bird again. Clipping their wings does not hurt them either. I also want to say that it is not at all safe to let your bird fly around your house at all either, especially if you have ceiling fans. Birds will hit windows thinking they are open, they WILL fly out open windows and can easily smash into walls and break bones. And it is very easy for a bird to fly on top of something very tall and get stuck, rather it be out of will and not wanting to come down or literally getting stuck. Ceiling fans can decapitate birds as well.
We had not clipped one of my bird's wings for a while (we had not realized this) and my brother brought her outside in the garage and while holding onto her completely, she flew out to the street. We were lucky she didn't get ran over and we VERY lucky she did not pick up the wind. The wind was crazy and it was a miracle that it did not pick her up and sweep her away. I also met a woman while I worked in a petshop who had just bought an amazon parrot and her daughter got her out of her cage and though the cage was no where near the door, she flew out the sliding glass door and they never found her. Another account, there is a loose amazon in our neighborhood. One of the neighbors who did not clip the wings had left a window open and when they were getting the bird out of the cage, it took off and that was that. Occasionally we see the bird fly around and it lives in the trees but it is not safe at all. The last thing I want to say in regards to birds that do not have their wings clipped is that it is so common for bids to snap their necks, especially the little conures, when hitting a wall or window because negligent owners let them fly around or due to not clipping their wings, they got out.
Ultimately it is your choice, though like I said, you have to think about the safety of your bird. Wings do not have to be clipped and should not be if kept in a large, outdoor aviary but if they are indoor pets, DO clip them.
After I clip the wings of my bird, I hold her feet and lift my hand she is on in an up and down motion. She starts to flap her wings, it is not only a form of exercise but it also a way for her to feel flight without being in harm's way.