A (usually large) proportion of customers will be confused and unintentionally pay for at least one month of full priced service: perhaps they don't understand they need to cancel before the 30th day, perhaps they forget to cancel, perhaps they try to cancel and fail.
There is a more honest alternative. Amazon Prime requires user input after the trial before their first $79 subscription charge is made. This is called positive option billing.
If you think you made the conditions clear but you get unhappy customers anyway, you've failed to communicate.
I used to have a subscription to a UK magazine where apparently automatic subscriptions are not legal, so they have to sell you the magazine again, every year. I thought this was great, as opposed to the situation in the Netherlands where you have to cancel at the right time, or else you are stuck with another year. (There are laws in progress to change this, so clearly many people are annoyed by it)