i was born and raised with out a gi, don't even think about trying to put one of those god forsaken things on me!!! take that, judoka.
With, and Why not? they Can offer the point of realism that like if you had jacket or suit or heavy shirt on. and hey are just more fun than average clothing.
The gi OR some type of upper body apparel can come in handy... Lapel chokes, sleeve chokes, extra grip (when you or your opponnent are sweating),etc.. It can also be used against you. I prefer both.
I'll just wear a dobok or just those white plain pants or black and fight without a top or wear those shorts like the Muay Thai/Kickboxers wear.
As a newbie in BJJ I find the gi to be a great learning tool...I get choked out all the time. for me to do anything, I need to do it correctly and I can't rely solely on strength to escape because of the grip advantage it gives opponents. everything is much easier when I take the top off and go with just a rash guard/underarmor.
Wander around on a friday night in a martial arts uniform, and you'll soon be fighting in a uniform. Personally, whichever.