Do you need a churchwarden? Of course not. But should you buy one? That depends on how bad you need one. If it's something you always wanted to try, be it for the cooler smoke or the channeling your inner fantasy LoTR character or your on a mission to some every style of pipe or you like the way they look or your particular lifestyle could use a long stem pipe for smoking? Then go for it! If you don't particularly care for them but feel it's something every pipe smoker needs, well if that's good enough for you then ok by me. But if the shape (length as well in this case) does nothing for you, then you can easily skip them and get something that makes you want to smoke it.
Just to throw something else out there, have you looked at demi wardens? Those stems like 7-9 inches, in between a proper churchwarden and a standard length pipe (whatever that means to you). I have a Danish that's a good 8 inches long, my Brigham Canadian is just about 7, and I have a couple long Forever Stems on cobs. I found that to be just perfect for me and my varying milage.