The more girls you have in your harlem will increase your income allowing you to buy books, affection items or quest cash check items and increase your stats. Lvling up all your girls will give you a bonus in stats and so many girls will change the rarity of items sold in some sections of the market. You do not have to collect them all but if you collect all the girls from the bosses that you can fight atm then you will remove them as possible drops from the epic prize machine, thus forcing it to give you prize machine exclusive girls. The 10x battle for 2 coins will not effect drop rates and when you use it a silver ball will appear and open if you got any girl or a legendary item, however the items will line up in a straight line if a girl has been obtained. Most event girls are placed on low lvl bosses so in the later game this will mean a slower cash flow if you grind for these event girls and a lot of them tend to be on dark lord. Also the more you battle a boss without getting a girl will increase the chances a little bit but not for event girls.