A tablet is a tool too. Its great for reading, reading e-mails, technical documents can be productive too. Its portability, form factor and battery make it great for special purposes (i use one for WMS puposes in the warehouse). Many commenters will also be able to enlighten you about how it helped them with their needs. And the portability advantages can outweigh the lack of keyboard disadvantages for some.
Its not because some people bought them because of a fad, many companies failed to create a decent strategy and app portfolio to aid their business processes and some people are too stupid to figure out their potential that you can generalise the uselessness of tablets.
And don't forget that a tablet can be a great tool for the purpose of entertaining yourself and fighting boring moments during waiting times (travel, dentist waitingroom,...)
(ps sorry for my english)