It's a learning curve. We, in general, have been spoiled by high quality, pixel dense displays. Going to the Rift can be a little bit of a let down in terms of visual quality. For most, the loss in visual quality is more than equally compensated for in immersion.
Rift really shines when it comes to formation flying, landing SU-30s on a carrier, in the future F-18Cs, aerial refueling and just general flight. For now and maybe a generation or two of VR displays monitors will offer better visibility outside of the cockpit.
When flying with others that are using a Rift, there's not much of an issue as everyone is on a "level playing field." There are advantages the Rift user has over a monitor user. For now the monitor user has a slight advantage when it comes to having the ability to spot the enemy further out.
With proper tactics that can be mitigated by Rift users...