Well, you can probably get away without it, but you'll be glad you have it when you see the gunk that it pulls out of the water!
You can get a very good skimmer for that tank for under $200 (I have the AquaC Remora--quiet and effective, around $170). There are several good models for closer to $100. I've heard good things about the Coralife Super Skimmer and the CPR Backpack, for example, and I think they're around this price. Even my old Skilter pulled some ugly crap out of the water, and they're like $40.