This really is not true all of the time. The tradeoff between INT and MAB is going to depend on a number of factors but the big deciders are Atma and tier of spell. If you don't have Ultimate, its going to offset the relative benefit towards the "2 int" end, and if you don't have MM somehow, its going to make INT a much more valuable tradeoff.
In all honesty, a better rule of thumb since the introduction of Tier V spells is (spell_M_value -1)INT < 1 MAB < (spell_M_value)INT, so for Vs, you're looking at somewhere between 1.299 and 2.299. If you are seriously MAB deficient (no beyond, baying, ultimate etc) you're even pushing the 2.8 mark. This blog
has a derived equation to determine the approximate value of an increase of 1 MAB, its useful for figuring out that kind of thing if you want to make separate macros / spellcast rules for different individual spells. Edited, Jan 29th 2011 10:15pm by NotJim