Believe me its probably much better that way. Unless the PLC is a big player in the market, the HMI vendor is not going to put out resources to fix your problem if one arises. Host can't afford to buy everyone's HMI and work out who's at fault when it doesn't work. Unless its all in one house, you get nothing but finger pointing. At least with Modbus being a generally accepted universal protocol, you have a chance to debug it and determine who to point the finger at. The user is caught in the middle.