Thank you for your answers.
I consider Kentico as an external dependency that's indeed required. But just like all other things that are not source files (CSharp generated *.dll, LESS/SASS generated *.css, TypeScript generated *.js,...) I don't think it belongs in source control.
By default our VCS excludes dll files from getting added to the repository and the repository size would grow significantly if we would add them.
Using NuGet would allow to restore (download) a centrally managed package which has a clear version number. Using a Lib folder, it's less guaranteed that a dll addition/update by 1 developer will end up in VCS.
Also I don't think we'll be replacing individual dll's, as we don't manage them (it's done by Kentico). If there's a change then there will be a new Kentico release (major/minor/hotfix) as a whole. (Mind that we are new to Kentico developent so I could be wrong on this point.)
See also http://ideas.kentico.com/forums/239189-kentico-product-ideas/suggestions/3898999-add-the-kentico-libraries-to-nuget