Heed this advice at your peril. I have very solid evidence that plurals and singular keywords require different ad copy and convert at vastly different levels. I also know that certain terms that otherwise seem exactly the same can have hugely different conversion rates.
I am not saying you need to track every single keyword, but in most campaigns less than 50 keywords drive a huge percentage of the volume. If you are not willing to set up separate tracking URL's for these words then you are probably not maximizing your revenue or profits.
Small subtle changes can mean absolutely huge differences in CTR and conversion. Not every keyword should be treated alike.
I have decided to take the quantity not quality approach. Tons of campaigns with slimmer margins will product more revenue as a whole for an affiliate then a few carefully tuned campaigns with meticulous keyword analysis.
If you focus too much effort on a few campaigns you'll max out at a few thousand a month. Its no secret that success in the industry comes with trial and error. The more campaigns you put up and test, the more opportunity for profitable campaigns you'll find. Personally, I have been much more successful with volume, I could careless if i squeeze the most out of every dollar spent. My time is much better spent looking for profitable campaigns. I run 500+ campaigns on 10+ networks, volume is key to my income.