Well the point is really this: If you don't need it, don't buy it.
The perfect scenario would be to make 1 arena grinder and 1 FG grinder (hell, they might even be the same deck), and just farm for the rest of your life until you get all of the cards/money you can. But that's obviously not going to happen. And hellfire's not going to rain down from the heavens if you decide to spend some cash on another deck, just to shake up the monotonous grinding.
Basically, just be smart with your money. And try to save up some of it in case you need it. I for one feel irresponsible if I go below ~45K electrum (which is a good money amount to stick around, as it means you could, at any given notice, fully upgrade a whole 30 card deck. I think Kev actually mentions this in the tutorial come to think of it..)