Yes, it does save power. If a device on your motherboard is enabled, it will draw power, even if it is not being used. Depending on the device and your power management settings, your OS may turn them off, or at least into a low power state.
As stated before, yes, you are using more power and memory. However, the amount is hugely insignificant compared to the use of the CPU, graphics cards, and even the fans.
Generally, if you arent using an onboard device, disable it in BIOS. This will prevent it from getting power at all. In addition to that, the OS doesnt have to load drivers for those devices, which uses a small amount of memory. But more importantly, in my opinion, you wont accidentally confuse one device for another, as it wont be visible to the OS.
Yes, you can leave them on with no detriment, but personally I say if its not being used, dont leave it enabled. You can always turn it on later.