In a word: nope.
dotmobi is simply in the business of trying to get you to buy something. There's nothing any more special about the .mobi TLD than
any other TLD.
To look at this another way: Would your website do better if you also bought .net, .org, .biz, .uk.co, .it, .cz, .ca, .de, .ly, and so on? Nope.
One of the best approaches to making your website accessible to mobile devices is to use
responsive design. Using CSS media queries, you effectively detect the width of the browser and apply sections of CSS accordingly. So, you're using the same content and just displaying it in a slightly different manner.
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