My understanding is that the minimum amount of credits that you can buy is 50, possibly because Pat doesn't want a bunch of people sending in just a couple bucks at a time or something of that sort. I imagine that that would become a nuisance. Another thing that probably has to do with it is that you can't buy anything for less than 50 credits at the CS. Of course, I don't know either of those things for certain. But considering that the free credit offers can give you less, I would guess it to be more of the first one. I don't think that people should be able to purchase less than 50 at a time from the actual credit shop because, despite their expansive onsite uses, are the way that the site receives financial support from users.
That said, credits are being traded a lot right now rather than just buying stuff from the shop. People are buying not only pets and items, but art, writing, codes, etc. For reasons stated by Uwi, people often want to pay in different increments corresponding to millions of tu, which is, obviously, a common increment on the site. Having the extra tokens at values of 10 and possibly 25 would help people to trade more exactly. Since 10 is equal to 1m, it seems like a good minimum increment.
Basically that was all a long winded way to say that I agree with making smaller credit tokens as long as the minimum amount for purchasing credits stays the same. Which may not have been a point I actually needed to make, but just throwing it out there since it is quite related.
Overall, support!