As for the “coupon sites” I wouldn’t exactly classify them as parasites. It is entirely the merchant’s choice: to work or not to work with them. If the merchant sees no use in promotion via coupon affiliates, they can spell it out in their program’s TOS, and enforce this rule.

If, on the other hand, the merchant does have room for coupons, but doesn’t want its customer to “leak” to coupon affiliate sites between adding items to their shopping cart and checking out, they can offer a coupon search tool within their own merchant site (Macy’s doing this).

I would also view other types of affiliates as parasitic (to a merchant’s affiliate program profitability):

Trademark poachers – As you have stated, affiliates using this practice are not contributing additional traffic to websites, but rather feeding off of traffic that is attributable to the brand/organic equity of that website.

Coupon sites – specifically I mean sites that solely promote coupons (and earn affiliate commissions through such coupons)
Coupon sites such as Retailmenot or Coupons4u typically don’t contribute additional traffic that would not have otherwise come to our website. Rarely do individuals go to a “big” coupon site, unaware of our site, and then click through to us. Instead, someone will get to our checkout page, see a coupon field, and then search for a coupon code in Google and find one to use from one of these sites. This leads to the affiliate receiving a commission for the sale and the customer receiving a discount on their purchase which, added together, significantly decreases our profit margin on those sales. One point of clarification – I have no problem with affiliates making lots of money, as we generally make lots of money too. When they don’t actually contribute additional traffic & sales, but rather feed off of our own traffic, I find it parasitic though instead of symbiotic.

One more important point to consider that could counter this parasitic effect of “big” coupon sites is the promotional effect of the coupons. Perhaps some of these customers would not have purchased had they not found a coupon to use? Our testing has found that few of these customers drop off completely without the promotion of a coupon, but that may be limited to our market. What have others found?

Another post on the subject coming up today. Some people just refuse to admit how parasitic behavior is actually hurting affiliate marketing, saying that by by slandering adware & parasitic affiliates I am “bringing us all down” making our industry look non-reputable [detailed wording here].