There is a thread elsewhere about this.
There I point out that this was the cause of a major affiliate uprising a few years back.
Some major sites of the times were stuffing cookies and ran paid ads for whatever getting people to cruise by their sites and stuffing the non-overwriteable cookies. That resulted in a monopolization of the market place – other affiliates were shit out of luck.
That is when it was decided that affiliates wanted to have the last referrer credited, making the cookies overwriteable and leaving the cookie stuffers without advantage.
Crediting the first affiliate encourages unethical practices by affiliates and is bad for the industry. It allows unethical affiliates to monopolize the affiliate business.
All of the old respectable brands lived through this and have overwriteable cookies.
Personally I will NOT promote any program that uses non-overwriteable cookies. I will not be doing all the sales for someone who just drives indiscriminate traffic and spews out thousands of cookies in the hopes that the rest of the affs will do his work for him.