mushy not bowling him is more of a reason to select a specialist batsman, if zia is bowling a lot of overs then his batting becomes a bonus but where is batting currently is he needs to be considered as a bowling allrounder, he can't be a batting allrounder his batting just isn't up to it. i agree that mushy not bowling him doesn't mean he's a useless bowler although there must be a reason why mushy isn't bowling him and if he was good or better than others then surely he'd be used more. looking at his FC stats says he is decent, looking at his t20 stats says he's not and looking at his list A stats says he's a decent/average bowler.
but where his batting is atm he needs to bat at #8 or lower imo and to be batting that low means you need to be more utilised as a bowler otherwise you're giving up a bowling spot for someone who slogs and doesn't really bowl.
All Time Test XI: 1 Hobbs 2 B.Richards 3 Bradman 4 Pollock 5 V.Richards 6 Sobers 7 Gilchrist 8 Miller 9 Procter 10 Botham 11 Warne