Personally, the rare farms do little good for me; I already have (almost) all of the (farmable) rares I could want, and I also have a high score (as I write this, it's almost 45,000) and plenty of electrum.
Nevertheless, while I can think of a relatively simple way to stop rare farms from working from here on out, I'm not going to say it. Why? Because I don't really think they should be gotten rid of. If I do need another copy of a certain rare, there's a good chance I can get it. Plus, there's the less advanced players to think of, the ones who don't have the resources I do. To be able to compete well in the high AI levels or in PVP, they need those cards, and while they CAN be gotten from AI's, it's hard to do, because the player has to luck into facing the right AI deck, beat it, and get lucky in the slots.
With the farms, that's easier. You still have to luck into facing the right deck, but winning isn't as difficult (to put it mildly), and the odds on the slots are higher. So, it's easier to get the rare you need, but not so easy as to completely defeat the purpose of the cards being rare.
Also, there is another benefit for us players who don't actually need the rares -- the "goldfish test". The term refers to testing a deck against an opponent that does nothing; it's a way to see how well your deck works without having to worry about the interaction with the opponent. This can often be informative, such as to help figure out what kind of hands you'll tend to get, how long you need to set up, and so on.