My portacot came with one of these and we did use it, but whether or not it's worth buying a separate one will depend on how much your going to use the cot I guess.

Once the baby can sit up you can't use them anymore. Our DD didn't sit up until 8 months old so we had use of it for quite a while, but most bubs can sit up much earlier than that.

The main advantage with them is not having to bend down to almost floor level to lift the baby in and out so it's really just a convenience/comfort issue for you, I don't think they make much difference to bub. Though I was happy to use the one that came with my cot, if it didn't have it I probably wouldn't have bought one.