The affiliate has a choice. They can either work with multiple merchants, or they can work with an affiliate network. So why should a marketer want to work with an Affiliate Network?
So how does working with an affiliate network prevent this from happening, and is this the only reason to work with an affiliate network? A good affiliate network has a number of different offers which the affiliates can promote, and nearly all the affiliate networks consolidate the amounts the affiliates earn from the various campaigns. This means that the affiliates receive a consolidated payment rather than many smaller ones. It also makes reconciling the payments easier, as you simply log into one affiliate network, and can see a summary of each of the campaigns you have promoted, how much you have earned from them, a summary of how much you have earned overall, and how much the affiliate network has paid you to date.
Also, once you work with one campaign in an affiliate network, the other campaigns work in a similar if not identical way - the tracking is the same method, the place you pull the creatives from is the same, the way you check your stats is the same - and nearly always, all of this can be achieved by logging into one backend site rather than many different sites.
So now you are working with one log-in, where you can really get to understand the system. However, even with the best of affiliate networks, things go wrong - but with an affiliate network, you would be allocated an affiliate manager, who will help you sort out any issues you encounter. Not 50 different affiliate managers - one for each campaign you are running, as would be the case if each merchant was a standalone merchant with their own affiliate program - but one affiliate manager for all the campaigns. And by building up a relationship with your affiliate manager, when you do need answers, you have one person to turn to.
Of course, once you become a professional super whaling affiliate (the type that lies on the beach while the money pours in - HAS ANYONE REALLY ACHIEVED THIS? Watch this space for a future story about the affiliate beach goers!), you'll need specific campaigns to promote, and that is why, even the best of affiliates belong to multiple affiliate programs and affiliate networks. But this presents even more problems.
So my advice when starting out is to choose one or 2 affiliate networks, become familiar with their system, build up a relationship with the affiliate manager, test out the different campaigns they have, and let them help you achieve the success and results you want to achieve.