I do still have a landline because I have to use a fax machine a lot for work and I work from home. It is necessary particularly for long distance faxes. We have talked in the past about eliminating the landline, but don't know how we would do it.
Also, we have three children and only one has a cell phone. One is 5. She is very familiar with how to use the landline phone (I know, I get messages all the time on my cell :) so it is helpful if the older two are babysitting and need to get me. We have 3 phone units in the house for emergency "just in case" calls when adults are not home and the 15, 12, and 5 year olds are.