I personally never used the mind flay glyph ever. Only time I ever wished I had it was on Heigan and I didn't loose much DPS, went from 3rd to 4-6th range on DPS meter.
I use glyph of power word shield instead. The heal and shield saves me time and again, and used to also be a big help on Kel'Thuzad when people would get frozen in ice.