Jim Storr's book as an excellent look at a whole gamut of military issues and he attacks a lot of things that military folks/observers take for granted. I'd have to check the reference (I don't have the book available to me now) but the context of Storr's statement is a look at organizing forces for optimal battlefield use. Storr is big on "operational analysis", using data garnished from conflict to inform military debate. Again, I'd have to check the source but I believe he is referencing some form of data (especially from the Mid-East) referencing tank kills to come to his conclusion. IIRC, Storr makes the argument to support his idea of formation level anti-armour organization.
Gudmunddsson's idea of the "sniper tank" MBT, the current incarnation of the MBT, in On Armor is also interesting. He posits that there is a bit of a dichotomy between "operational mobility" and "tactical capability" (for lack of a better term) and that modern Western MBTs have emphasized the latter at the expense of the former in the last generation of tanks. Almost all development of MBTs has been on targetting systems and, to a lesser extent armour, at the expense of operational mobility. Modern tanks are now "Sniper Tanks", meant to win battles in the first shot (unlike WWII where, usually, tanks would have to smack multiple rounds into eachother to win) but with horrible operational mobility (fuel milage, durability, etc).
That being said, the average speed of the F echelon of an Armoured Division was the same in 2003 as it was in 1944, so perhaps operational mobility is moot?
As for an answer to your original question, I've seen two "purposes" for MBTs. In irregular warfare, we use MBTs because insurgents are often unable to really do much to them; we used Leo2s there for a variety of missions when we felt like making a statement. They're essentially moving pillboxes.
In a more regular setting, there may be validity to the statement that "tanks aren't the best at killing tanks", or at least that they aren't the most efficent method of doing so. In a doctrinal area (static) defence, tanks aren't usually parcelled out in one's defensive area to kill enemy AFVs, but are rather tasked as countermoves/counterattack.
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