McNair opposed heavier tanks because he was trying to get as much army transported to the combat theaters by the very limited amount of shipping available. He also didn't want assets tied up by organic assignment to units which didn't need it all of the time. He felt that proper recon could identify masses of enemy armor and that centrally assigned TD could then be massed against the tanks. When stuff ifs permanently assigned, it is hard to get it centralized. When it is centralized, it is much easier to attach it out or to "put it in support of". Did all of MacArthur's divisions need organic TD battalions? Did the Europe divisions need TD battalions on a day-to-day basis?
In digging, I've come to find it is much more complicated than that. Fundamentally, NO ONE in the War Department really much wanted heavy tanks, but they were blindsided by Roosevelt, just as with the aircraft and antiaircraft issues. In May 1940, Lynch the Chief of Infantry, expressed interest in a heavy tank to augment the light and medium designs and to keep up with the Joneses (i.e. the Germans). It led to the development of the T1 series, but after the Armored Force was created the interest went away and in 1941 it was only proposed as a pilot run of 50 for the US Army and 50 for the British. Then on 3 January 1942, Roosevelt announced he wanted 500 heavy tanks by the end of the year, so much of 1942 was consumed in figuring out how to make the T1 work and get it into production, only to have the end users all announce they could care less. So very little did McNair really have to do with it.
The issue of "heavier tanks" as opposed to heavy tanks came up later in 1943, when the Engineer Corps got the AG to freeze the weight limitations set in AR 850-15 to 35 tons for tracked vehicles and limited the acceptable width of vehicles. It got the Navy vote too, since they had programmed the designs of the LST, LCT, and LCM based upon 27-ton medium tanks and 40-ton heavy tanks, since that was what the Army had told them...back in 1941 to plan on. It also affected the design of the Liberty and Victory ship winches and cranes. Again, little to do with McNair.
He also had little really to do with TD doctrine and weapons design, which was the purview of Bruce and the TD Center...his input was most felt in 1941 as the "antitank expert" at the AT conferences, but then he became fully involved in creating and training all ground forces. In which he over and over tried to rein in the excesses of the TD Center's organizations...with good justification - antitank commandos anyone? Yes, he preferred towed guns over SP artillery - and TD's were SP artillery - but then, all US Army Artillerymen tended to prefer towed guns, well into the 1970's, which was a holdover from the earlier prjudice for horse versus mechanical draft.
He also had little patience with "perfect solutions". When Jake Devers did his about face regarding the 76mm-armed Sherman, McNair had no compunction about turning that then scarce resource over to the TDs, who said they would make use of it. The delay in fielding a 76mm-armed Sherman was the fault of the Armored Command and Devers, not of AGF and McNair.
And yes, attachments for TDs made sense. In the same way, it was decided that a specialized "jungle" division for the Pacific made little practical sense, which was good since it allowed standard divisions to go to either theater with little advance notice. OTOH, while MacArthur's divisions did not need TDs on a daily basis, it turned out that the ETO and MTO divisions did benefit from regular attachment...the problem was that not enough were available, just as enough tank battalions weren't available, because it was believed the pooling principle would obviate the need for a one-to-one ratio of those battalions to divisions.