Theres a number of reasons - mostly because it would be bad for a brand to sell IEMs for music use with a single balanced armature. The reason why I would think ACS do it, is that they provide comms units for motorsports - so for that sector, where it's just radio comms with the pit, a single balanced armature is good enough for speech where hi fidelity isn't that crucial.
Second - a sleeve adds length to the amount that an IEM sticks out of your ears. The ZS10 is quite a beefy enclosure as it is. Couple it with a sleeve... well, it's not going to that inconspicuous - but more so than a pair of cans.
I've worn over the ears cans on stage. You have to pay a lot of money in IEMs to get similar sort of bass performance to a set of cheaper, good sounding cans. All depends upon how self confident you are ... or whether the aesthetics would be a problem for the band or not.