Question: have you ever trained in something besides wrestling before? Just an honest question. I wrestled before I started bjj, and did no-gi for my first year, and I paid for it in the long run by developing technical flaws and bad habits. I didn't want to train in the gi because I thought it was stupid, but now that I train in both 50/50 I love it and I see the benefits.
First of all a lot of people train for sport competition in the gi.
The gi increases friction and forces you to be more technical, especially with escapes. No gi has a lot of sweat, and allows some leeway in escaping positions and submissions, which means that you will often neglect proper technique in favor of explosiveness, which works well at the lower levels but not so much against higher levels. I believe the gi also helps your base and balance, and increases your knowledge of leverage, but that's just me.
Furthermore for self defense purposes, people don't walk around in skintight rash guards and grappling shorts. Neither do they walk around in a kimono, but they wear clothes (like in NY). So from that perspective it's best to train in both.
I'm not biased against one or the other. I train about equal in both. But I think both provide benefits, depending on your goals.