When people use those blocks it's because they're trying to build a script, a sequence of instructions. You can't do that with reporters, so this would make it harder for people who only know about programs as a sequence of steps. They'd have to use IGNORE all the time.
People who know about reporters shouldn't mutate lists as side effects of computing a function. You make a new list (which might or might not share storage with the old one, depending on how you make it).
If I were designing Snap! from, uh, nothing, I'd have just one block shape, so you could use the same block as an input to another block and as a toplevel block in a sequence. They'd look kinda like those Darwin bumper stickers. But I'd still encourage people to program functionally, without side effects.