It does work, however, it's extremely unethical. Not so bad if you go on intuition and feel, but once you bring scales into it and you start weighing it's... argh.
It is heavily frowned upon in hobby shops, especially if they're nice enough to have an open box system where you can pick them out of the box yourself. Heck as a collector even I frown upon it. Weeding out every holo packet in the box is unfair to others who wish to also purchase cards. Even in department stores, if they do open boxes, it's still going to look really bizarre if you start weighing them all (and again, remember, while a lot of adults play it your average store teller is going to consider it a children's game and again will consider it unfair if you start weighing packets)...
Which is why most stores don't carry open boxes like that anymore, they get shipped blister-sealed boosters which add even more weight and make it harder to discern the weights. If TPCI keep the current pack rate they've introduced into HGSS (making it more like Japanese packet ratios) then you won't need to 'weigh the packs' since 1/3 packs actually has a holo in it now, and usually a rare one while we're at it. From what I've read the BW packs in english are pretty easy to pull FA cards from, so you shouldn't be needing to weigh them.
The reason most people buy large numbers of packs and haven't pulled foils is usually because of someone buying all the weightier packs before them, frankly. Something I have had the displeasure of seeing in person.
Sorry if I went a bit ranty, but it's a topic which really grinds my gears.