Voucher code sites depend on affiliate commissions.
The reason they are not popular with other affiliates is the 'last cookie gets the sale' rule which means if a customer goes through lots of affiliate sites before purchase the last cookie set gets the commission.
When a customer finds your site via affilaite A and decides to purchase he or she may see the voucher box in your checkout and think, "I'll hunt down a voucher," (this is actually quite effective and you will often find discount codes doing this - a good tip).
The customer will pass through the voucher code site and even if you do not have a current voucher code current a cookie will be set, overriding the original affiliate A who worked hard to get you the sale. The voucher code site will get the commission, at the very last moment, after the sale is already in the bag.
It is alledged they cynically create pages for websites knowing voucher codes are not available and force visitors to click to reveal a code which doesn't even exist, thus setting the cookie.
We also discovered our 'best performing' affiliates were voucher code sites and researching deeper revealed the commissions were being awarded after the sale was in the bag and so we now do not work with voucher code sites. We did not loose business as a result.
We do offer voucher codes from time to time but only direct to customers. They can get listed on places like money saving expert, but that is fine as long as you aren't paying out twice, one discount and one commission.