No, you don't really need one, but sometimes it can make life a whole lot easier. If a friend has one, borrow it. It makes calibration faster, it makes making copy negs, or internegs, faster. It makes the science end of photography easier. If you have an interest here, then go for it. You don't have to spend much money to get a nice one. I use PMK, too, and I don't use a color densitometer. I use an Xrite 301 with a blue filter. This is the same as the blue channel on a color unit. This gets one pretty close and as close as you can get with any densitometer. To get closer you would need to run spectrals on all you materials and then you'd be way off in the deep end. When it comes down to it, it is only the final print that matters and not whether you have a densitometer.
I was just looking at ebay in closed auctions and noticed that there are many units that closed for under $100. I got mine for $30, as is, not working. Turned out to need a fuse and an aperture. Xrite will sell apertures, but cost more than I paid for unit, so I made one. I pull it out and use it about once a month.