Building a Mini-Site
The reasoning behind building a mini-site is this: Google doesn't have an awful lot of 'respect' for a single page landing page that links to an affiliate link. So if a keyword is worth targeting and you plan to stay in the niche a while, why not create something that will give you a better quality score and lead to some organic search traffic. One way to do this is through a mini-site.
A mini-site is a site with a relatively small number of pages but is designed to look to Google and humans alike like a website as opposed to just a single page. The quality score that your ads get depends upon many factors, one of which is that your site should have links to other pages. That's difficult on a single page landing page because you only want one link on there.
Your website real-estate
This is how I 'manage' my affiliate marketing: I start by using quick methods to assess a niche - set up a campaign with a few adgroups and get a feel for the keywords. Quickly optimise my keywords into tightly focused groups and weed out any poor performers. Then I'll try to let the campaign run for a quick measure of profitability. If it looks like it might be profitable, and it's a niche where there are naturally related products that I could expand into then I might consider doing a mini-site.
Part of the reasoning is that if it looks like I might be sticking with this product for a while then that gives my mini-site time to gain traction in the search engines. If I didn't think I would be staying in the niche for long then there's often not much point building a site.
I wouldn't like to say what the strategy should be for certain because it varies from niche to niche. For example, if you know the product is going to have a short attention span from the market then it's not worth putting any effort in; do what you need to do to make as much profit as possible quickly and then get out. If a product looks like it will be long term but also might attract a lot of competition and you want to stay in, it might be worth building a site as soon as possible to give yourself time to build up the content in a natural way, gather back links and so on. This is so that as the PPC becomes more competetive and more expensive, you'll hopefully be starting to get some ranking in the search engines.
Google is always going to favour a site with even a few pages over a single landing page. You have to balance whether the amount of work is justified.