I cant think of a time were I had this issue.. I have not tried with my 2015 15in rMBP, but I've used my 2011 17inch MBP for years (obviously lol) and never noticed it drain while plugged in. I've used hand brake, heavy gaming, parallels emulation gaming and it always stayed charged. Are you guys using the 3 prong power cord that came with the MBP or just the 2 prong? The 3 prong delivers a bit more power than the 2 and could be your problem.