If you have paid for ATR42s or can get dirt cheap leases (the RE ATR42s are all pretty knackered early built -300s IIRC), it makes sense to keep them if you don't need more than 48seats. If you purchase new equipment, it does not really make sense to waste money on an ATR42-600 unless you operate into restricted airfields (like, for example, LIAT in the Caribbean).