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Thanks for the reply. As managing web production is one of my responsibilities reducing the number of 404s for a 600K site has priority (for me - hence the question). Our company has 40 plus active campaigns and countless ad groups - and therefore thousands of display URLs. My motivation in asking this question is to reduce 404s -  if I can convince our paid search staff that there is value (other than less work for me) to following a best practice, then I might expect a degree of buy-in on their part. As Pashima alludes to below, from a conversion standpoint making your landing page relevant to your ad and search terms would seem the goal for a company like Google that professes to be all about the UX. not to mention your own internal ROI tracking.

Exact match type efforts are great.  For a single phrase.  Or longer tail phrases off of that.  As soon as you want to get ranked for variations that in their sum total require going beyond the exact match, they're not as valuable.  Especially if you need to start from scratch. On the other side of the coin, is all of the content on the existing site high enough quality AND truly relevant to your goals?  If it' snot and you need to scrap it, you're going to take a hit on those statistics anyhow. And on a final note - don't trust the PR numbers alone.  They're inaccurate at best, and at least months old.  Could very well be that the real PR (not the TBPR you're going by) is lower, especially after Panda/Farmer. Better to look at the traffic statistics on the site both current and historically compared.  If you don't have analytics to review, it's a crap-shoot.