Firstly, there's nothing wrong with buying gold-filled or plated jewellery but it's important to understand what you're buying so you don't get ripped off.
Many of us prefer the colour gold to silver, but not the price tag. Gold is incredibly expensive right now, approximately 6x more expensive than gold filled per gram, so jewellers are trying to find more ways of making their jewellery more affordable and plating opens up a bigger market.
However, I see a lot of filled and plated jewellery being sold under the guise of 'gold' and many people don't know the difference. Gold plated is either silver or cheap base metal that has been given a fine coating of gold, the thickness of which is measured in microns. The actual gold content usually represents less than 5%. This wears off over time and can leave the base metal exposed. The speed at which this happens varies depending on the thickness of the coating, the amount it's worn (daily wear is likely to accelerate the process) and how often you polish it. You can of course always get items re-plated but tot up the cost of doing this a few times and you may as well have invested in gold in the first place.
'Filled' is a relatively new term and one that is causing a little confusion. Filled, like plated, isn't solid although the gold bonding is much thicker. This is made using a mechanical heat process and legally must contain a gold content of 1/20th of the total weight. It's much thicker than plating, so much so it can usually take more wear and tear, even the odd knock, without exposing the metal underneath. Gold filled is generally more expensive than plated, but cheaper than solid gold.
So, what does it matter?
Well apart from the obvious price difference, you are not buying something that will hold its value in the same way. Yes, gold-filled can look just as beautiful and no one would ever know by looking at it, but it's not the same and will never be worth as much. It's unlikely to be melted down in years to come by your granddaughter and reformed into something they would like to wear. The cost of melting and refining the minimal gold would probably be more than it would ever be worth. So, whilst filled and plated is a more affordable way of getting the look you want without the price tag, just be aware of what you're buying. If something is advertised as gold but cheaper than its competitors then you should be questioning it. If it seems too good to be true it usually is.
Everything I make is made with the intention of it becoming an heirloom of the future, which is why almost everything I make is with precious metals and semi precious stones. Yes, its more expensive but not only will your jewellery hold its value, but it will be able to withstand the next few hundred years...or until your granddaughter melts it down.