The major arguments mounted by art historians against his theory fall into seven categories: (1) artists did not need to "cheat" because they were highly trained in drawing from observation; (2) artists did not need lenses because they were so talented; (3) such devices would have been too cumbersome; (4) no written proof, from artists or others, exists that lenses were used; (5) artists could have used a grid instead of a lens to get the perspective right; (6) the lens hypothesis has been overstated; and (7) even if true, it is of no interest to art historians.
Now, I do find myself wondering how fair this summary is. Are art historians quite as silly as (1) and (2) make them sound? But let's not quibble too much for the moment.
The problem with numbers 1-4 is that they fail to rule out the use of optical devices. Whether or not artists had the skill and/or training to draw without lenses, whether or not the lenses were cumbersome, and whether or not anyone at the time wrote about them, artists still may have used lenses. The arguments about training and talent are also inconsistent with the general acceptance by art historians that Renaissance artists used geometry to draw in perspective...[and] sometimes used tools such as strings, grids, and planes of glass... to get the perspective right. The problem with the grid argument is that the use of a grid might explain how artists got the perspective right, but not predict the smoking gun, the errors.
For my money, after reading the details of Falco's argument, it seems to me that it's that last point that stands out as the most compelling point in its favour. But I'm no scientist. As a historian, however, I don't think that (4) can be written off that easily. Yes, it's true that silence in written sources does not rule out the use of lenses (and arguing too much from silence is dangerous); but it does seem odd. Is there no documentary evidence at all? (Not even anything nicely ambiguous?) And, importantly, how does that compare with written evidence for other innovatory practices of the time? If important developments - including those listed - were usually recorded by somebody, then the silence really needs some explanation. It might, indeed, be related to (6) and (7), which seems a particularly crude reduction of an important issue: it isn't entirely unreasonable to suggest that a technique by that went unrecorded and was completely forgotten about (suggesting that few artists ever used it, perhaps) may not be that historically significant. Is it?
Still like the theory though.