Things we buy and use all fulfill a need in our lives. I can clearly identify why I need a telephone, and why that telephone needs to be mobile. I can also clearly identify why I need a computer, and why that computer needs to have a decent screen size and a keyboard. But I can not identify any functional need in my life, for which a tablet would fill that need. This is nothing like the option "need fulfilled by other devices". Yes I have other devices that fit identified needs, and yes a tablet can also fill some of those needs in a limited capacity, but there is absolutely no need that I can see for a table.
As an example, lets look at the basic need of human communication. This can be broken down further into two sub-needs; speed and reliability. Verbal communication takes care of speed, and written communication somewhat addresses reliability (repeatable, retrievable later, reviewable, etc). To meet the need of fast communication throughout history humanity has expanded the distance over which this need can be met by creating and improving tools such as signal flags, signal fires, telegraphs, radios, television, telephones, close range mobile phones, longer range cellular phones, and other such tools. Each of these tools have also been used for other, similar purposes (example: fax machines, entertainment, SMS messaging, etc) but these tools are still focused on the primary need of fast human communication over great distances. With tablets, I see they are trying to add such additional purposes, without fist being a tool to meet a clear primary need.