Ar7 is not necessary but there are some cool ar7 maps out there that some people (like myself) enjoy playing. The only thing I could think of that would make ar7 "necessary" is for low ar HD which is an important skill in the long run for anyone looking to be a versatile/complete tournament player. If you don't have any plans to play in tournaments nor enjoy the experience of playing on ar7 though, then just stick to higher ar stuff.