Having to grind for a month is bad design though. Really, this exploit is good for pvp.
Well bumping this as today I had 4 out of 5 random games as farm. - But really, THAT IS THE POINT OF RARE, you're not supposed to base decks on rares, they ought to be a certain novelty you only get once you "completed" the rest.
The type of "novelty" rare you describe only exist in nymphs and marks. These cards are indeed novelty cards that should not be based around in decks normally and the reason they are so sucessful is that they con only be obtained via the oracle or PvP leagues.
And here lies the problem of shards and weapons: they can be grinded for. Because they can potentially be obtained in a relatively short period of time if one puts the effort in, many people feel the pressure to obtain shards and rares or they feel they miss out on some important play strategy. On the other hand, since nymphs are truly rare cards and can only be obtained in time, there is no such pressure.
Thus the obvious solution to the farming problem is to explicitly make shards and weapons "true rares" like nymphs, or normal cards buyable from the bazzar or somehow more easily obtained through other means than PvP farms. If people has easier ways to get rares than farms, then problem solved. Trying to make shards and weapons "semi-rare", less rare than nymphs but more rare than normal cards is the root of the problem.