they're quite common actually. though the other slot is x16, it has x4 lanes. the advantage is you can buy a x16 or x8 RAID card (for example) and it'll fit nicely. not many motherboards have x8 and x4 connectors... so putting a x16 connector is a good solution. raid cards don't need more than x4 bandwidth.
i wish more botherboards had an extra x16 connector w/ 4x lanes. that way i can run SLI and have a high performance RAID card.
my concern is that even if the pipes are reduced to 8 per card, i'll only pump 4 on the x4 connector. If that is the case i'd like to understand how much of a performance reduction, if any, it will be.
as you can see there are minor differences in speed between x4 and x8. however, keep in mind they're testing the 6800 series cards. the performance gap has likely increased quite a bit since then, especially with the latest 79xx series and definitely with the 88xx series.
my advice: don't bother with SLI on a x4 connection.