I'm still confused as to what on earth "efficiency" has to do with whether we should tighten regulations on the use public assistance. "Sure, he spent his food stamps on hookers and blow, but all economists agree that this was way more efficient than if we had restricted the use to beans and rice." What is your point?
doing inefficient things is stupid, even if the inefficiency makes you feel good.
Your logic is impeccable. Doesn't matter if public assistance is spent wastefully, just as long as it is spent "efficiently." Booze and cigarettes for everyone! 
do you have evidence this is a major problem? I mean based on a system-wide study, and not based on your grocery store anecdote.
Nope, though I haven't really researched it, either. Just a lot of anecdotes and common sense. Can you know of some objective studies to the contrary?
Well now you've got me curious. Why isn't there more data on this? Turns out, federal rules actually
prohibit reporting on which stores (convenience, grocery, etc.) take in the most SNAP, and what the SNAP is spent on. So, the nest I can find are observational studies, like this one, which estimate that roughly $2 billion of SNAP is spent annually on sugary drinks.
http://www.doctorslounge.com/index.php/news/hd/32079Still waiting for an answer on why Michelle Obama wants to take vending machines out of the schools and send notes home to parents who aren't packing a healthy lunch, but doesn't say a word about how SNAP is spent?