I usually one get the ones I like dues to my limited cash flow.
There have been only a few exceptions.
Like for example the Universe Special Editions. I like the look of the package as a whole, with the figure sealed inside. It looks really nice. Hence I aim to get them all and keep them MSIB, and if I really like the look of the figure I'll get a 2nd to play with. I have done that with Megatron.
Another example would be if it is cheaper to get an extra one since it is part of a wave or bundle or what ever.
Animated Jazz was an example of this. I only doubled up on him because it was only a little bit extra to get Wave 4 as a set of 4 instead of the 3 others (EG Bumblebee, Sentinal Prime and Swoop)
Otherwise I just can't justify it. I don't have enough money to buy things I don't like.
I always likened it to buying as many video games as you can and not playing most of them