You've caught my bias (but probably a legitimate bias) towards the all plastic "breeder's box. Too many females and fry die in those of shock and/or asphyxiation.

I'm not trying to be too mean towards those shop owners. It is a tough competitive business and if they get stuck with some product which is a clunker, they still need to move those on and make shelf space and get their investment back. Hopefully they will not order more. :)

Cheapskate aquarists like me do need small, coverable containers to hold fish being caught and while they are being acclimated to new quarters. Small soap-less condiment jars, covered with a small yogurt cup (the yogurt was eaten, Daphnia cleaned the scraps off of the cup) work fine.

The "breeder nets" do exchange water (and oxygen/carbon dioxide) with the rest of the tank and presumably with the surface. Though the female guppy is constrained, there must be some sense of contact with the rest of the fish in the aquarium.

They are also useful in segregating vulnerable fry from potential predators the tank. However, if there are larger fry eaters in the aquarium (angelfish for instance) it is rare but not unknown for fry to be sucked through the netting and eaten!

I sometimes have also put gravid female livebearers in covered drum bowls or gallon widemouthed pickle jars in water from the main tank (and ideally have changed 100% of that water out daily, refreshing the water supply with water from the "mother tank.") Indeed I was pleased to see Stan Shubel doing much the same with his breeder females because he is the sort of aquarist I certainly look up to.

Also have collected and hatched killifish eggs in small plastic containers and started the fry in 2.5-gallon tanks. Initially the close quarters (if the easily upset water quality is closely watched) is beneficial to the fry. Food, especially live food, is "right there" for the youngsters. They even have small sponge filters (and at one time there were bowl-sized under gravel filters) for the mini tanks.

See sponge filter H0. In this case the company listened to the hobbyists and developed the product.
http://www.jehmco.com/PRODUCTS_/FILTRATION/Hydro-Sponge_Filters/hydro-sponge_filters.html

But sometimes we (and I do mean we) are operating on a false economy of space because we don't have room to move those fry to grow out in larger aquaria. 10 and 20-gallon tanks come to mind here.

Almost no aquarist who gets into breeding fish [how arrogant that we call ourselves "fish breeders", the fish do the breeding, we do support services ;) ] has enough space. But if we are going to save fry and are serious about raising quality guppies, we need to plan on giving a batch a 10-gallon aquarium pretty soon. And at a month, if we are really going to control who breeds with whom in order to insure size, color and quality of finage, we need to separate those little females out as soon as they show their gravid spot.

I remember how astonished I was in my student days to meet a Wisconsin guppy fanatic who maintained a great line of half-black red delta-tails. He had a dozen aquariums dedicated to just that line. It turns out that while few aquarists (a fraction of a percent) would have that many tanks, those serious about raising superior guppies will dedicate six to even that many aquaria to a strain. Now consider that they may keep several strains...