I disagree. I think that having the option to buy for 1 month would definitely increase sales rather than decrease them. Players that are already buying in 3 month lumps will most likely continue to do so. And this will allow new players that want to try supporter for a bit without paying too much to do so. Also, you say that 12 dollars isn't much to kids, I agree that it isn't much, but how many kids have credit cards or online facilities for payment? They have to ask their parents, and parents are more likely to say yes to 4 dollars than 12 dollars.
Also, 12 dollars isn't much to us Australians, but it might be a lot to someone else in a different country where the exchange rate is high. It might cost someone a lot more to buy supporter than we imagine.