There are a lot of misconceptions about what Google is saying about mobile SEO. First and foremost, Google doesn't mandate the use of Responsive Web Design (RWD) as best practice for SEO. Google expressly says "Google does not favor any particular URL format as long as they are all accessible to both Googlebot and Googlebot-Mobile” the bots Google uses to crawl desktop and Smartphone specific content. And it’s worth noting here that Google is crawling desktop and mobile content separately. Secondly Google unequivocally says upfront “Google supports smartphone-optimized sites in three configurations”; those being RWD, content adaptation, and separate mobile sites.
Google says it recommends RWD for smartphone sites “because using a single URL for a piece of content makes it easier for your users to interact with, share, and link to your content, and a single URL for the content helps Google's algorithms assign the indexing properties for the content". This is often taken to mean that the only choice is between Responsive Web Design (RWD) and a "separate site" when these are not the only options. You can easily have a “single site” that uses device detection, just like Google and many other large companies do where content adaptation is done server side to maximise the speed and quality of the experience delivered to the device. Google itself doesn’t use RWD for any of its core products. So what Google says is not always what Google does.
RWD (where sites are adaptive on the client side), is not always the best option, either from the perspective of bandwidth efficiency or mobile SEO. For mobile SEO, the main problem concerns the indexing of the mobile version of the site. Building a responsive site which adapts to a device with a full browser, just does not guarantee your site will be indexed by Google as a mobile website. If the mobile version has the same URL as the desktop version, it may simply be ignored by their mobilebot search engine. And this is the nub of the issue, the mobile version of the website may not be indexed as a mobile site and will not be ranked as such (as highlighted here by SeoMoz).
Keyword research is crucial for SEO. But this cannot be controlled with a responsive website because your site will use the same keywords for both mobile and desktop. For a site designed specifically for mobile use, it helps to have different keywords and content on your mobile pages to meet the unique mobile goals of the site.
So while on the one hand Google talks about its preference for responsive Web design, on the other their Webmaster Guidelines advise us to “think about the words users would type to find your pages, and make sure that your site actually includes those words within it". From an SEO standpoint, that means optimizing both sites with different keywords – difficult to do if you have a one –size-fits-all responsive strategy. You can check this out yourself in the Google Keyword Tool. If you select “All mobile devices” in the Advanced Options and Filters you'll get keywords from tablets, smartphones and feature phones. Google Analytics also gives an overview of keywords used by mobile and tablet users. Ultimately, the best mobile SEO strategy is one that brings most qualified traffic. If you want to optimize your mobile site for different keywords than your desktop version, using two different URLs indexed for different sets of keywords is best. In other words, separate sites with separately targeted keywords is a better approach for the average site. If you want to show the same content without any separate optimization for mobile, then one URL will suffice.
As for the debate around "single URL vs. separate mobile URL", it’s clear that having a mobile URL will not affect a site’s organic search traffic because Google has introduced a method for differentiating between mobile and desktop. The right URL will always be shown regardless of the device used to access it. Check out this video where Matt Cutts recommends using different URLs for desktop and mobile websites.
[Graph by Guy Podjarny from Akamai]
Building a site using responsive design can take significantly more time than building a separate mobile site. If you’re set on RWD, you should consider using the technique alongside server side device detection (RESS) where knowledge of the device can be used to decide if an efficient responsive experience should be sent to a device. We advise that you build it mobile first and then work up from there. More details and reviews on all the techniques are available here.
In summary using RWD should be planned according to your business objectives: if your website has a lot of content, if users do not expect anything different from the mobile version and if you don't have any expectations in terms of revenues and loyalty, then maybe responsive design might be the right choice. But if you want to allow people to have a good and customized user experience, if you want to allow users to make the choice of whether to view your mobile website or switch to the full website version, or if your website is highly interactive and you want it to fulfil different goals for mobile users, RWD is not always the best choice. To provide that level quality control over the content delivered, you need to understand the underlying device upfront.
So while it may be easier for Google to crawl responsive sites, they’re not the only option and are certainly not the best option for mobile SEO.