I respectfully disagree with your assertion, from my point of view skinning has always been a strong point for mojoPortal if you have skills with CSS and html. One should not confuse their own weakness, ie lack of knowledge/skill with html and css as a weakness in mojoPortal. mojoPortal has always been very skinnable and almost any design can be achieved.
HTML 5 and CSS 3 require just as much skill/knowledge if not more than previous versions of html/xhtml.
Artisteer is a design tool that generates html and css so it makes it easy for people with less skill/knowledge of html and css to make designs. There is no change in this, we will continue to support Artisteer because it is the best tool I know of for people to make designs without having much knowledge/skill with html/css. Even people who do have skill with html and css may find Artisteer useful for rapid design and prototyping.
Artisteer will also continue to improve and future versions will output html 5 and css 3 instead of xhtml 1.1 that it outputs now. Actually Artisteer just put out a beta of Artisteer version 4 a few days ago that supports html 5 and css 3.
Artisteer is also putting out a new SDK that "may" make it possible for me to implement a plugin for Artisteer that can directly export a mojoPortal skin. I will be researching this in the near future. If we can make that happen it be even easier to use Artisteer than it is now.
Actually it has been somewhat possible for a while to use newer html structural elements in mojoPortal but the ways to configure that were not documented (and still are not fully documented yet) but the new html 5 example skins we ship illustrate how to configure the use of html structural elements in layout.master and in theme.skin. But still to make your own html 5/css 3 designs requires skill and knowledge of html 5 and css 3, so these example skins are mainly helpful for people who do already have those skills and knowledge.
LESS is a programming language that allows use of variables so you can change the variables to output different versions of CSS, but use of LESS requires strong knowledge of CSS. LESS is really just like CSS but with variables and functions, and less must be converted to pure CSS before the browser can use it. For people with knowledge of CSS, LESS provides some opportunities to do cool things. You can define colors and widths as variables for example then you can change those variables to produce different designs, ie similar in some ways but different colors widths etc based on the variables, so you can get some re-use of things easier with LESS.
There has never been a better time to learn as much as possible and develop skills with html and css, these are the essential skills for serious web designers. You can get by without it to some degree using tools like Artisteer but anyone who really aspires to be a web designer should be learning html and css.