My point regarding double domains is that the majority of your traffic is coming from Google, people are going to look at the title and maybe the description and then decide they want to go to that website. Unless your website has a logo that says the whole domain then people generally don't care what the domain name. So they finish browsing the site, a few days later they come back and want to go back to the website - they can remember the company name but not the actual domain name. They start typing in your company name and assume it is .com, .co.uk or .net. Yes they can go to Google and serach the company name - however I assume you are going to use a double domain because the domain you want is already taken for .com, .co.uk or .net - so therefore they will most probably come up before you on Google.
Its just a theory but that is my reasoning behind not using a double domain, I don't find them bad per say, I just believe is should really be a last resort behind .com, .net, .co.uk, .org. etc.