They give you more thrust for less effort. Why NOT use them?
I don't use mine when they would be too long for the planned dive - e.g. I'm going to go inside a wreck.
I also might not use them if I am planning to take pictures and I anticipate that the specific conditions and environment will require the kind of technical control I get with short, stiff, tech fins.
I also don't use mine in water that is cold enough that 3mm neooprene socks are not warm enough.
But, warm water, open ocean diving, they are great!