Talk about one-strand fiber links at full-duplex

I suppose that you are talking about WDM (Wavelength-division multiplexing) using SFP. The bandwith available for a single strand of fibre is bounded only by the money : quoting a figure from a telecom vendor 80*40Gb on a fiber for a transport network dwdm device.
Using cisco as a reference , i find that the maximum bandwith available for non-telco dwdm is 1G ( 10G coming ). The advantage for the provider is that using DWDM it doubles the available optic fiber for connections. For the consumer , there is no advantage nor any disadvantage , since it is 'transparent'.

The two solution already mentioned (DWDM & single-core-SFPs) are both methods of getting multiple signals down a single strand of fiber.

As Haakon said, single-core-SFPs work in pairs and just replaces your normal SFP/GBIC modules in standard network equipment. And as cavver said, DWDM is another method. DWDM though is more complex, requires separate pieces of equipment and is much more expensive.

The advantages are that you can get more data down the same infrastructure allow you to double (at least) your bandwidth. [DWDM gear can carry multiple signals (160+ channels) over a single piece of fiber and can intermix data and voice traffic.]

The disadvantages are you may sacrifice the distance the signal can travel and of course you increase your risk. If a single strand is cut, twice as much is impacted.