With all of the dual lands, tri colored decks aren't impossible. For EDH, I lean towards three colordecks for the exact reason G Moto states. It gives me options for solutions to multiple opponents. There are still some weaknesses (
Grixis is notably weak on options for protection from Enchantments), but in general you have many more options. In Standard, I lean towards 2 colors more because I enjoy building decks along a theme. While I wish they would have banned the pairing of Saheeli and
Felidar Guardian (I am scrambling to make the infinite aether loop replacements needed for the loss of
Felidar Guardian), they made their decision. The deck can still consistantly end gane around turn 6 or 7, but a 6 cmc
angel is still a delay from a 4 cmc cat. Still, I enjoy brainstorming ideas, so toying with working red into the green white deck. Just not seeing enough options from what I currently have available.