I can see that for primary keys like "lead id" the "is in" option is useful. but when the key is a string (like product code) it is useful to be able to use filters like "starts with" or "contains.
Similarly, i can see other scenarios where you might like to use "is in" with a non-key field. e.g. a customer sets up a custom field where they enter a value that means something to them 0,1,2 or A, B, C. they may want to use "is in" here.
So, i think both are useful, i just think we should "merge" them together so that you can use "is in" when you want and can use "normal" filters when you want too.