If you are talking about the hearing-impaired assist mix, that's probably going be a heavy on the vocals and key solo instruments, and very light on low frequencies. It could be a useful source to mix with an audience recording with low vocals, but it will be very compressed and limited already, so a delicate touch would be advised.
ALDs and IEMs are very different beasts. ALDs are typically just a mono soundboard mix with some EQ/limiting applied, whereas an IEM is artist specific.
Of course the compressed audio can be expanded, but it really would help to know the exact algorithm that was used, and the manufacturers ain't tellin'.
The post above nails the biggest concern, and that is that each IEM is meant to subtract the user's contribution to the mix.
I've heard some full-bodied, otherwise great sounding recordings of the Dead that may lack one drummer or the other, or perhaps lacks a guitar or keys.
There were stadium shows where I remember an occasional lot-rat with a scanner listening the shows which he didn't score a ticket for.
That was before digital radio, code and frequency hopping, and other security measures were used to secure the signals from illegal users and to prevent security radios, taxi radios, cellphones, CBers, etc. from jumping onto the mixes.