Dear Michael and others,
I think that the most likely reason why yeast methods suggest this concentration is
that 2 % agar is less easy to "overcook" (i. e. partilally hydrolyze during
autoclaving), especially in the slightly acidic yeast media. So, the real purpose is
not making the yeast happier (yeast does not care - it grows as well in liquid) but
to make you happier by ensuring that your plates are not too soft.