I think you're confusing a few things.
First, system RAM is used if the chipset has integrated graphics, and is not dependent on the type of RAM used. This is the only situation where the motherboard will have the option to set aside system RAM for video memory.
Second, not all add-in graphics cards will allow you to siphon off the system RAM, and is usually the lowest-end cards. I'm not sure anything from an 8600 series and up allow for this. Even if it does, it does not require the graphics card and system RAM to share the same type of memory.
It's like saying since the US uses gallons, we can't make use of containers that go by liters.
System RAM and the RAM on an add-in graphics card are independent of each other.
For the best video card, a more powerful graphics chip with less RAM is a better choice than a weaker one with more RAM. A weaker card that has more RAM doesn't have the horsepower to make use of the higher detail and screen resolutions that would make the extra RAM effective.
Don't judge a video card by the type and amount of RAM it uses. There are low-end cards with 512MB that will get creamed by a mid-range or high-end card with, say 128 or 256MB of RAM, even if the low-end card uses DDR3 or DDR4 and the other uses DDR2.