I don't see the play until next week, but in-ears are increasingly common onstage –– more so for musicals (to hear the band or costars or a click track), but not unheard of for plays too.
If it's a high-tech play, he may need to do or say super-specific things at an exact moment.
If it's being used to jog his memory on an as-needed basis, that's really not anyone's business. Most actors would love that safety net, especially in previews when things can change multiple times a week. And it doesn't sound like RDJ's stumbling around on the level of Pacino or Bruce Willis, who clearly didn't have a grasp on the material (though now we know that Willis was dealing with the early days of dementia).
Think most people paying $100++ would vastly prefer this situation over, for example, Elaine Stritch's refusal to use an in-ear in her later years, leading to borderline unintelligible and uncomfortable (tho still iconic) performances.
Theatre adapts with the times. Those unwilling to embrace change should find something else to occupy their time!