Of course there is, as the market has clearly shown. I suspect that there will be a market for 5", 7", 10", 12", and so on tablets. Consumer choice is good.
Yet your post seemed to posit that because the iPad has succeeded where the Tab hasn't, every decision Apple made with the iPad (in particular form factor) has been validated. That is specious reasoning.
It should be the case that iPad is the market leader in this category. They created the category and made a very nice device. The other tablets are not nearly as polished as the iPad and this is obvious once you use them.
Concluding from what I've written that I think that "every decision Apple made with the iPad (in particular form factor) has been validated" is a miscarachterization.
However your original statement didn't have a lot of substance beside calling Apple's competitor's a "joke", and I took from it what I could glean.