It will depend on how much you use a phone.
I don't need to make many calls and don't enjoy talking on phone, so for me it is a no-brainer. I have a very small LG flip phone with Verizon prepaid service. Great coverage, and my typical monthly expense for it is from $5 to $8.
I currently have an iPad 2 and a 5G iPod Touch. The iPod Touch is a great "substitute" for a smart phone when you have wifi available. The new Google + Hangouts allows you to easily place and receive phone calls on it with a Google Voice number. Works great. All this would also work well with an iPad Mini, but of course much larger.