David Airey's blog has an interesting post in Logo trademarking tips: A legal perspective
But, before addressing the legal implications, it’s worth noting that a number of our insightful readers and commenters have already helped articulate a variety of pros and cons from a business and marketing perspective (view the comments on Duets Blog). By my count, there appears to be consensus on at least two important points:Logo trademarking tips: A legal perspective | David Airey, graphic designer
1. Having an iconic stand-alone non-verbal logo or wordless trademark symbol is highly desirable, especially for truly international brands; but
2. be prepared to spend a lot of time, effort, and significant resources to achieve one.
The combination logo is your best bet for trademarkingIt may also be worth taking a look at "NO FRIGGIN CLUE" Confusingly Similar to "CLUE" for Video Games, Says TTAB from The TTABlog
Generally, this format and style is more flexible, easier to clear for adoption and use, easier to register and protect each element separately, and easier to enforce rights in both verbal and non-verbal elements.
With respect to enhanced flexibility, a trademark owner can elect to always use the verbal and non-verbal elements together, perhaps as a way of reducing the risk of infringing on another’s prior rights in a mark perhaps similar to either the verbal or non-verbal element.