It's not even needed for PicKit3, as I've been using MPLAB with a PicKit3 for a while now.

My reasons for starting to use MPLABX are that it runs on the operating systems that I prefer (FreeBSD if I don't need to use the programmer, Linux if I do need to use it), and that it is the future for Microchip. It allows me to retire a computer that I use only for programming the PICs.