I dont think meditation would render PB useless. Meditation would specifically be used pre combat and in phase transitions where you cant hit anything, while pb would be used to get your stacks back when you lost them due to a mechanic but are still able to attack things (not attacking for 8 seconds would be a big dps loss with or without GL3).

Also keep in mind that pb is much more than just a way to get GL3. Rockbreaker spam is actually stronger than doomspike due to GL3. I think meditation would give pb more room to be used creatively rather than you having to use it at the start of the battle and then save it for the phase shift.
Granted there isnt much gain for using it for singletarget situations currently but that might change with new skills in HW.