WiFi is so much faster than 3G unless you have a slow internet connection at home. My home broadband is 10x what I see on 3G.
I think it is just a plan they can force on people that would not use the internet anyways as increased profits for phones. 25MB is very limited. I think email only would work as long as you never opened attachments.
I guess for me, the wi-fi/3G issue is really a non-issue. If I am at home (where I have wireless-N), of course I use it -- FOR MY LAPTOP and DESKTIOP. But if I am at home, I do not browse the web on my smartphone.
The purpose of the smart-phone for me is portable internet for when I am NOT at home. And then, 3G is awesome, because it is always there, and you do not have to hunt for a free wi-fi connection.
But back to the original question, I think those 25MB plans are kind of a joke; as you will almost always go over the limit in a month, if you use the web browser at all. I remember having that in the first month I had my old Treo 700p. And within 10 days I switched to the unlimited plan.