I agree with the User Ferret above - the words used to define genre boundaries are somewhat arbitrary across many forms of media, particularly music and video games.  Are "action games" the only games with action? "Strategy games" the only games with strategy?  "Role-playing games" the only games where you play a role?  As Dota 2, Heroes of the Storm, and League of Legends all share the same basic gameplay and deviate sufficiently from many other games, it makes sense to use a genre label.  MOBA is the term I have heard from at least 100 sources whereas ARTS is a term I'm hearing for the first time here.  ARTS is ambiguous.  Is it something related to "art games"?  If it stands for "action real-time strategy", is it an action game that requires strategy and is not turn-based?  Does Dragon Age fall into this ARTS category?  MOBA is a much more widely-accepted term describing a very specific type of gameplay, despite the vagueness of the acronym.  Someone who is aware of MOBA games will instantly what the term means when given examples of MOBAs.