The skim remains controversial. Crema has always been viewed as the best indicator of a good shot of espresso. I would encourage one to view a blog post from the coffee collective:
James Hoffman gives a wealth of information as well, so check out his blog too:
I’m definitely not going to argue with anyone who loves their crema. Crema can give the espresso body and definitely gives the espresso a beautiful color. However, crema can also give the espresso bitterness. I suppose it also depends on the sort of americano you are looking for. Some prefer to have their americanos resemble a ten ounce mug of drip or french press coffee. On the other hand there are those that prefer a cleaner, but softer, americano; the first skimmed americano I had resembled a fruit juice. We have conducted different triangulations involving skimmed and unskimmed americanos. Consistently we have found the cleaner americano to be the skim. I recommend the skim, but as mentioned before, I won’t force a skim on anyone. Furthermore, I do not recommend the procedure for any americano larger than eight ounces. To anyone that has received a skimmed americano and didn’t ask for one, we apologize, and to anyone who didn’t enjoy the skim, we apologize. But to those that loved the skim, boy isn’t that tasty?