In addition to what Bob mentioned, being able to use data for free over wifi is a must. Understand that 'unlimited' data is not really a literal situation. With all of the prepaid providers (MVNOs), beyond a specific usage point you are often throttled (e.g. speeds dramatically slowed) or cut off entirely (less common). Not only are many wifi connections faster, but using it when available helps you stay inside a more reasonable limit to prevent the slowdown.
We just got our first smart phone iphone 5S from Sprint. Had a Net10 pay as you go phone before. No contract but $30 for unlimited phone & Text + $30 unlimited data. I connected it to my wireless router. What's the advantage of doing that if any? Is it better to use wifi when you can?