You can't home grow schwag. Schwag is nasty because it is poorly mass grown in a big field, smuggled, aged, and compressed. During it's journey to you, it can undergo many temperature changes, as well as extreme temperatures. It can be smuggled in a gas tank. It can be stored on a farm next to donkey shit, and absorb the smell. By the time it gets up to some cities in the USA, it can be 2-3 years old. Very little respect is paid to product quality by anyone who handles it in that period.
Freshly grown weed from schwag seeds can be as good as any of the best, but there is no guarantee. You have to plant MANY seeds, to increase chances of getting a vigorous female. Either way, your freshly grown bud is fresh. THC degrades over time.
Good genetics are always better; planting bagseed can be a crapshoot. You never know how long it will take to flower, or if it is disease resistant, or mold resistant, or a vigorous grower, or potent, until you grow it. When you buy a seed from a reputable seller, you know what to expect, and it's going to be a strong grower, and potent, no gambling.