There are so many devices these day. We can distinguish those into four categories: desktop, laptop, tablet and smart phone.
Do we actually need all of them?
In my opinion, we don't.
I think one laptop and one smart phone is enough. Nowadays, laptop has great performance and high resolution screen which means desktop doesn't have any merits. And also if we have laptop and a smartphone, we don't need any tablet. Tablets has very ambiguous needs. It's heavier than smart phone, but smart phone can do exactly same thing with tablet.
For me, I'm using Mac book air and iPhone.