They are not a neccesity if your cat doesn't care to use one. I have a couple cats that are not interested in one at all.

There are cats who are horizontal scratchers and cats that are vertical scratchers, just figure out which you cats prefer.
cardboard horizontal scratchers are great, as are sisal vertical posts.

Just beware of picking up a used scratcher, they can carry disease, such as ringworm and distemper.