They are calling this "the only phone you need" because it will work well inside your home where cellular reception may be poor and outside. Calls are unlimited and the cost is fixed.
If you use your telephone for applications other then telephone calls, it may not be the "the only portable device" you need -- they are selling telephone service, not open Internet connectivity.
Would you consider this service yourself? (Check the prices before answering). What do you use your telephone for beside phone calls? What might you use it for in the future? How did you like the T-Mobile web site?
The New York Times covered the announcement in this article.