It is a good way to collect cards, however it seem that it is on par with practice mode and solo arena. You can grind practice mode with a decent deck you will lose 1 in 10 games or less, this way you get about 250 soul gems an hour. I was able to get 1200 gems in about 3-4 hours. The rewards from solo arena are the same like the vs arena minus some gold (20 I think). To get enough gold to play arena at all you need to get 150 gold, this takes time which in theory you could use to generate gems from practice. So overall if you look at the time invested if you play only expert practice mode 3 hours per day everyday for about 2 weeks you will have all the cards you want guaranteed, while on the arena method you will have more cards but don't get to chose which.
Also I am sure people have manged to create macros that beat the 5 soul gems AI by now for sure, this seems very easy to abuse at this point.