According to the state deer biologist I asked at an IDNR meeting a couple of years ago the reason for the antlered only firearms licence is primarily a herd reduction strategy. Most deer hunters are extremely buck oriented, and the greatest number of deer are culled during the firearms period since that's when the most deer hunters are afield. The reason the other periods are either sex is to encourage more people to hunt those periods, which is also why crossbows are allowed during late archery. The bonus antlerless license is $24, not $35 as stated earlier, and you can use it during the firearms period. If you aren't shooting does during the firearmsperiod you're not reading your hunting guide, you'll find it here:
http://www.in.gov/dnr/