I actually agree with ChopperBill. That is one of the ways I conserve my data plan. If I can download things while on free WiFi, that I know I want to read later in the day, I do. That doesn't stop me from using data when I need it.
I have two ways to connect to the internet that I pay for. One is relatively cheap through my wifes iPad and the other expensive through a pay as I go iPhone plan. My laptop can only connect through the iPad. So my first choice is free second iPad third pay as I go and I try to balance need versus want for each one.
Now even though the iPhone is my most expensive option I have it set not to download attachments so it is a good way to preview eMails and delete junk before downloading to the computer. Also the mobile versions of many web sites use less data, so even though the phone is 10 times more expensive to use it is sometimes cheaper.
To further save data I have my computer based mail setup not to auto download attachments bigger than a certain size, my mail allows me to leave it on the server, I then download it when I'm on free WiFi. I also never watch any videos unless I'm on free WiFi.