Just thinking about it:
Direct linking - each click onto a PPC ad is a potential sale
Landing page - only a click onto a link on landing page is a potential sale
With landing page you have to calculate your own click conversion (clicks onto PPC ads vs. clicks onto tracking links), plus, the conversion of the merchant.
Make up scenario - 100 clicks onto an ad, converts into 50 clicks onto tracking links (landing page click conversion 50%) which then convert into 1 sale based on merchant's conversion of 2%. This means that direct linking would produce 2 sales.
Furthermore, those days, building landing pages for PPC (read AdWords) is tricky and may lead to QS problems which are not just low QS anymore, but put people into trouble with account terminations.
From this perspective, and unless you're promoting some fishy stuff, you're always in a better position with the direct linking then your own landing page.
Any decent merchant should have no problems with the QS.
Finally, time to develop and maintain a site (landing page) can be better spend onto worrying about PPC side only.