The entire concept of IEMS presenting a hearing-saving opportunity is based on the sound isolation they provide from stage/venue volume.
Assume that your IEMs have a great seal. You might receive 20+ db of sound attenuation. 95db stage/venue volume (is it really that low, ha ha?) - 20db attenuation = 75db. In that sense, there is still ambient bleed of 75db.
Supplement ambient bleed by adding volume to targeted sounds of importance in the IEM mix. Hopefully, the resulting IEM mix is still much quieter than stage/room, assuming judicious personal IEM mix volume.
Pull an IEM out of one ear for whatever reason, losing IEM sound isolation, and all bets are off. As stated by Pacman, many players match the remaining IEM volume with the room, say 95 db or more, in a sealed, close proximty to the ear drum. Depending on the frequency ranges inside the IEM, and the dynamic spikes of IEM instruments, the IEM level ends up being equal to, or greater than stage/room levels.
If the room is acoustically poor, slap back of FOH sound to stage can have a discernible negative effect on musician timing if one IEM is removed. Latency and phase cancellation are often added to a mix that includes using just one IEM. So much for having a tight band in this situation.