That isn't new to SW2015. It's been that way for years, maybe always but I started with SW2009 so I don't know about before that since 2012. (Edited, and thank you Kelvin)
It's a desirable feature since people (me included) often sketch half of a symmetric body, and then mirror the sketch or body, or use the sketch for a Revolve. etc. In these situations we usually know the full dimension and by using that feature we can enter that dimension without the need to divide it by two. Plus that way the sketch will show the full design intent dimension.