that is true. it is about having a low unsprung weight on the suspension. especially real alloys. not just a shiny rim. look out for original bbs, enkei, oz racing and the likes. usichezwe na mushaino from china. these can even withstand a bit of rough treatment
They are good for passenger vehicles because they are light. Even if they are lighter by a single kilogram for each wheel it still makes a big difference. One kilogram inside the car is not equivalent to one kilogram on the spinning wheel. A kilogram in the car is almost insignificant but on a wheel its hell of a difference. a kilogram spinning at high speed becomes something else. Steel rims are durable and you can even decide never to slow down at bumps and you wont break the rim, but you can break something else.
The problem is cheap alloys look nice when they are big in diameter. Small but smart alloy rims are expensive(simple, one color, and with slender curved spokes. About 10 or 12 spokes). Then if you increase the rim diameter you might have to reduce the profile(height of the tire side wall). Once you reduce the side wall, your car feels like pharaohs wooden chariot in 3000BC on rough patches of the road. That's one of the reasons why those German vehicles with 17" rims cannot match the comfort of a JDM car sporting 14" rims on Kenyan roads. So you can only add up to one inch to the rim if you already have a large sidewall without reducing comfort.
Shida ingine iko hapa. Mimi nimechoka kutafuta. If you are looking for a alloy rim for a car with a small engine, you might buy the same size but one that is heavy and meant for a car with much more power. This is so in Nairobi where the sellers and customers are clueless. I went around town and the rims i was getting were chunky and obviously heavy despite being OEM (made just for looks). The chinese ones might not even be aluminum-magnesium, but some iron coated with some shiny material. If you increase overall weight of the wheel the car will become slower and use more fuel. If you reduce you get economy, nice pick up speeds, and the car handles nice; inatii kabisa.
I am tired of searching for light wheel brands like Eco Forme, BBS etc. I am now looking online but I know shipping will be costly. meanwhile, sina alternative. You have to get that kind of an alloy rim to make a difference.