The body can be maintained by any kind of foodstuff, but cow's milk is particularly essential for developing the finer tissues of the human brain so that one can understand the intricacies of transcendental knowledge. (Srimad Bhagavatam 3.5.7, Purport)
So if you get milk products, milk, then you can prepare so many preparations full of vitamins, which will nourish your brain. Dull brain cannot understand what is spiritual knowledge. Therefore, that Mr. Bernard Shaw, he wrote a book. Perhaps you know it. You Are What You Eat. If you keep your brain dull, then how you can understand? Because without becoming very intelligent man, one cannot understand Kåñëa consciousness. That is stated in the Caitanya-caritämåta, kåñëa yei bhaje sei baòa catura: "One who takes to Kåñëa consciousness seriously and perfectly, he must be very, very intelligent." Dull brain cannot accept it. Bahünäà janmanäm ante jïänavän mäà prapadyate. So we have to make our brain very clean. And for that purpose you require to drink not very much, at least, one pound or half-pound milk daily. That is essential. (From a lecture by Srila Prabhupada given in Hawaii, January 15, 1974)
One pound of milk is approximately equivalent to 2 cups of milk. Srila Prabhupada is recommending we drink one or two cups of milk every day. This was spoken mainly to an American audience at that time and Prabhupada was well aware of the cow slaughter industry. Again, Srila Prabhupada is recommending milk consumption (albeit polluted) on the practical grounds of developing good intelligence. In other words, Prabhupada is correlating intelligence with milk. Prabhupada also is talking about cultivating intelligence for attaining spiritual knowledge and use this knowledge for preaching. So if the end purpose is to preach Krishna Consciousness with proper intelligence, then according to Prabhupada, it is ok to drink one or two cups of milk daily (not necessarily more than that). Utility is the principle.
In Sri Isopanishad, Prabhupada talks about “making best use of bad bargain”. Polluted milk is a bad bargain but we can make best use (utility principle) ONLY IF we cultivate intelligence to spread the movement of Krishna Consciousness. The means of milk consumption (1 or 2 cups) should be to the ends of preaching Krishna consciousness, otherwise not. This is how I understand the above two quotes.