Even Marcelo Garcia, arguably the greatest no-gi grappler in the world, regularly says that he believes the gi is essential to building a complete game. It highlights gaps in your game, and tightens everything up, forcing technique over power or explosion. Then again, guys like Jake Shields rarely/never train in the gi, and nobody questions his legitimacy on the ground. So I'd say it's not mandatory, but definitely has strong benefits. My feeling is, since it helps build endurance anyway, do some time in the gi, and after a couple years, if you feel it's not helping, pack it away.