I think most people on the site would agree that you need a knife as part of your PSK. The type / size will depend on scenarios / use. However, even when I've got a survival knife with me, I still carry a small lock knife inside my PSK (and my urban kit for that matter). Theese knives don't necessarily need to be expensive and glamorous (that's what your main tool is for) but they can still give you decent back-up capacity. I always include a Whitby Firefly in my kits purely for redundancy. Your profile doesn't say where you're from, but I'm sure you'll find something locally.