I am a server, as is my husband, and I believe it will hurt them. We took an envelope of cash last week for our trip entitled "gratuity" solely for the purpose of tipping in all areas, but there were a couple servers we didn't leave over and above 18% for because they just didn't give the service. If I would have had to pay for it all out of my pocket, they were still good "enough" for 18% but in a couple cases, just not worth the additional funds.
Maybe it was because of included gratuity that they gave a lesser service, but for me, as a person coming with extra gratuity to spend, and as a server myself, I think it's the wrong attitude.
In the case of next year when it's all in the guest's hands, I believe servers will suffer.
You are going to have people who don't realize it's in their hands, people who are just bad servers, people who "punish greatly" for bad service, then you are going to have people wanting to skimp to make up for ALL the money they are spending in different areas of the World, then you are going to have people who didn't mind throwing a couple bucks on top of the included 18% that can't afford to tip 25% completely out of their own pocket for every meal, and lastly, you are going to have people from all over earth that just don't even know what an average/good/bad tip is.