A quote from Isaac Hopkins, which I agree with:
"Queen Excluders... are very useful in queen rearing, and in uniting colonies; but for the purpose they are generally used, viz., for confining the queen to the lower hive through the honey season, I have no hesitation in condemning them. As I have gone into this question fully on a previous occasion, I will quote my remarks:—
"The most important point to observe during the honey season in working to secure a maximum crop of honey is to keep down swarming, and the main factors to this end, as I have previously stated, are ample ventilation of the hives, and adequate working-room for the bees. When either or both these conditions are absent, swarming is bound to take place. The free ventilation of a hive containing a strong colony is not so easily secured in the height of the honey season, even under the best conditions, that we can afford to take liberties with it; and when the ventilating—space between the lower and upper boxes is more than half cut off by a queen-excluder, the interior becomes almost unbearable on hot days. The results under such circumstances are that a very large force of bees that should be out working are employed fanning-, both inside and out, and often a considerable part of the colony will be hanging outside the hive in enforced idleness until it is ready to swarm.
"Another evil caused by queen-excluders, and tending to the same end—swarming—is that during a brisk honey-flow the bees will not readily travel through them to deposit their loads of surplus honey in the supers, but do store large quantities in the breeding-combs, and thus block the breeding-space. This is bad enough at any time, but the evil is accentuated when it occurs in the latter part of the season. A good queen gets the credit of laying from two to three thousand eggs per day: supposing she is blocked for a few days, and loses the opportunity of laying, say, from fifteen hundred to two thousand eggs each day, the colony would quickly dwindle down, especially as the average life of the bee in the honey season is only about six weeks.
"For my part I care not where the queen lays—the more bees the more honey. If she lays in some of the super combs it can be readily rectified now and again by putting the brood below, and side combs of honey from the lower box above; some of the emerging brood also may be placed at the side of the upper box to give plenty of room below. I have seen excluders on in the latter part of the season, the queens idle for want of room, and very little brood in the hives, just at a time when it is of very great importance that there should be plenty of young bees emerging."—Isaac Hopkins, The Australasian Bee Manual