Dota 2 is the #5 PC game being played right now according to Xfire and is only growing. It's pretty unreasonable to call it insignificant.
from WP "According to Valve's founder and managing director, Gabe Newell, the company's investment in Dota was sparked from the collective interest of several veteran employees, including Team Fortress designer Robin Walker, programmer Adrian Finol and project manager Erik Johnson, all of whom had attempted to partake in team play at a competitive level."
So it might be a popular game, but it's not a major development for Valve in the sense that Team Fortress, Portal 2 or HF2 was...and also demonstrates some of the internal culture issues Valve has.
In the case of Dota 2, the game is bringing in millions via the in-game store. The game has 200k active players at any given time, and they're well on the way to 1mil active players in a few years. A significant portion of those players are buying tournament tickets. Most of them are buying custom equipment like couriers, announcer packs, etc.
Dota 2 will be the most lucrative Valve project since Steam. To call it insignificant is to be short-sighted.
If you still think the MOBA genre is insignificant, then perhaps this infographic will offer some perspective: http://www.forbes.com/sites/jasonevangelho/2012/10/12/league...
Source: I worked in the MOBA genre.
What do you consider a major development? Sure they hired the team behind the original but they still developed the game and put it out with company resources.
Team Fortress was a mod for Quake before HL1 was a twinkle in Gaben's eye. TFC was someone saying "that sure was a fun gameplay mode, but I can't get anyone to load Q1 any more... maybe I'll recreate the same thing over again"