To my mind, the question was never if the use of antibiotics to promote growth was a good idea, but rather -- if the practice is widespread. And in fact, it is. With the bulk of livestock producers in the US using antibiotics in this manner, it would be surprising if beekeepers didn't also try it. And maybe there is some noticeable effect, which hasn't been fully explained. All the same, the trend is away from non-therapeutic use, as it should be. See:

Non-therapeutic antibiotic usage in food animals creates resistance genes, promotes antibiotic resistance
Published on November 16, 2011 at 11:34 PM
http://www.news-medical.net

In a review study, researchers from Tufts University School of Medicine zero in on the controversial, non-therapeutic use of antibiotics in food animals and fish farming as a cause of antibiotic resistance. They report that the preponderance of evidence argues for stricter regulation of the practice.

The use of non-therapeutic antibiotics is widespread
• According to estimates, antibiotics are eight times more likely to be used for non-therapeutic purposes than for treating a sick animal.
• As much as 90 percent of antibiotics given to livestock are excreted into the environment.
• Alternative farming practices such as reducing animal crowding, improving hygiene, and improving use of vaccines have been shown to compensate for some of the growth benefits conferred by non-therapeutic antibiotics.

[ note that even in this critical review they refer to the "growth benefits conferred by non-therapeutic antibiotics" ]

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