Quite common. A friend of mine did exactly that. He has a GTX 1070 and I recommended to him a
Nixeus FreeSync capable 27" 1440p gaming monitor. He bought it and used it at fixed 144 Hz. Why? because FreeSync capabilities cost nothing extra and the monitor otherwise is an excellent gaming panel. So now he might get the added benefit of being able to use FreeSync (Nixeus already said they're looking into it) with his NVIDIA card. He was not locked in to anything because that cost component nor exclusivity component of FreeSync was ever there. He got exactly what he paid for originally and this announcement is just icing on the cake.
Prior to this announcement, the only way to get adaptive sync on NVIDIA hardware was by paying the GSYNC fee. Look at it this way (assuming NVIDIA card):
1) pay GSYNC fee to get adaptive sync support.
2) pay for a fixed sync monitor without adapative sync support.
3) pay roughly the same as #2 for a adaptive sync monitor but no GSYNC support.
#1 is vendor lock-in
#3 makes the most financial sense
AMD cards only ever had #2 and #3 as options.