Not wanting to put words in the OP mouth but...it depends - as a Gold with Qantas (Sapphire) allows you to use the AA lounges on domestic flights and if you fly often with Qantas it allows to UG using miles.

Also Qantas potentially takes far less miles per annum to reach Emerald than AA, so if the amount of flights p/a drops there is still the possibility of keeping Emerald with one airline.

Having said that I use AA miles more than anything as I always have to get out of their poor long-haul business class and into a bearable first class on trans-pacific flights.

It has to be said, for me, keeping three on the go I find the most wasteful part is the time I spend working out which program to put each flight and faxing across and chasing all the one's I chose not to designate before I flew.