I look at it this way. Let's say I come across Acme Corporation's website. They are selling what appears to be a really cool product. I figure I will give it a try and signup for the free trial and at the same time enter my credit card info. I give it whirl, but in the middle of testing it out I get sidetracked with a phone call or whatever and forget to finish testing. I wasn't impressed enough with the little testing I had completed to go back immediately and after a couple days I completely forget I even signed up. Now 29 days later my credit card gets whacked for a $20 monthly fee. I have two choices at this point. I can file a chargeback, which isn't fair to the company. Or I can eat the cost and cancel immediately, which isn't fair to me.
In my opinion, there is absolutely no reason to demand a credit card upfront for a trial.
*Edited to fix a typo.