There is not a single concise, satisfying answer that we may give you.
Each line in the CSS file is there for a reason. From a historical point of view, those lines would tackle browser bugs in more than one way. For instance, a floated element contains it's floated elements.
The floated elements contain their floated ones; also their total width is the sum of the contained elements width and not the entire line; to name a few reason why there is a float.
I can assure you that here in Telerik we are working constantly to improve the controls in both terms of performance and ease of use. The front end teams are examining the CSS files and removing redundant CSS or updating them when possible.
Currently we can not estimate when and if we are going to drop the "float: left" line. But when we plan to do so, there will be a notification in our blogs prior the release of the Q, as this would be considered a breaking change.
If you would have any other question regarding the RadMenu, I will be happy to help you.
the Telerik team