The mini is one of those devices that doesn't make sense for a lot of people, but for some it is an essential form factor.
If you had another iPad, it probably makes no sense to have/get a mini. They both do mostly the same things. Put another way, if you have a mini for some reason, another larger iPad likely makes no sense - go for a MacBook Air instead, so you have a device that differs significantly for the extra weight/size.
But if you need something that does iPad type stuff and an iPad is too big for whatever reason; its a niche that nothing else fills.
I loved mine (original iPad mini) when I had it. I used it more frequently than my iPad 4 until it got too slow - I had both due to some special circumstances (both work supplied devices). So much lighter, easier to carry in one hand, etc.
But yes it is definitely a compromise and if the size/weight reduction isn't important to you it makes no sense. But as above... in some circumstances the iPad is simply too large, and for those people it doesn't matter what the cost or processing power difference is - the iPad won't fit.