Why is full-hinting for fonts not on by default?

I'm running 10.04 which uses libfreetype6 version 2.3 and which is not able to do proper hinting of certain fonts. However, if I understand correctly, later versions are capable of hinting fonts properly. So why does Natty still have slight-hinting by default?

It's a matter of taste. Full hinting can be very aggressive, as it changes the original shapes of the glyphs to fit the grid. Some people like this XP-like crisp look, but others prefer slightly fuzzier Mac-like style. There has to be some default in Ubuntu, and slight hinting sounds like a good compromise.

Because the patent for font hinting expired just recently and the old version of libfreetype simply isn't compiled with font hinting.

Plus because Ubuntu doesn't provide major software upgrades within one version, you can expect font hinting only in the upcoming Ubuntu releases (talking about the official packages of course).