Sail boats need some sort of lateral plane to keep them from blowing sideways when the wind pushes their sails. They also need some weight down low to keep them from tipping over when the wind pushes up high on their sails. In many designs, these two functions are provided by a keel or keels.
Twin keels provide both those functions, and allow slightly shoaler draft than an equivalent single-keel version of the same hull. They also allow the boat to go aground safely and comfortably, so that drying berths and bottom painting on the beach become sensible choices. They generally have more wetted surface than the single-keeled equivalent, and will almost inevitably have more frontal area. This leads to their primary disadvantage: they will usually have poorer performance in light airs than their single-keeled equivalent.
It isn't surprising that cruisers and racers differ on whether that light air disadvantage is outweighed by the several advantages. The surprising thing is that some long-term cruisers are firmly in the racing camp, and believe that upwind performance in light airs is far too important to compromise for merely practical considerations like the ability to find a berth in a crowded harbor, or the ability to clean the bottom at low tide, without needing a grid.
That rule about poorer performance in light airs isn't invariable: Brent Swain's designs use the same quantity of plating to make the keel(s), whether there is one or two. The single keel versions have a fairly long keel, and the twin keel versions have two fins. Both boats use the same hull, and the only difference is the shape, number and placement of the keels. Since the one or two keels are made from the same plate, they definitely have the same wetted surface area. If he were to design a version with a single fin with less surface area than the present keels, one would expect the lesser wetted surface to mean better performance in light airs than either of the current boats deliver. Because this hypothetical boat's ballast could be deeper, it could get by with a bit less, giving it a bit more useful payload, too.
Why hasn't Mr. Swain designed a fin keeled version to get that little bit of extra performance when the wind dies down? Maybe because the fin keel boat would be far more awkward to cruise in. It would have to have a deeper draft than either the full keel or twin keel versions, it would be far more trouble to put aground for bottom cleaning, it would lose any advantage from wave cancelation that the twin keels might provide, and the only gains would be the ability to ghost along slightly better in light airs, and to point slightly higher into the wind. More to the point, Mr. Swain's boats have heavy displacement hulls, and are designed to be built of steel. If performance in light airs were important to the owners, they wouldn't be building that sort of boat, regardless of the kind of keel.
Given that most cruising boats have a motor, often with a large aperture cut in the hull near the rudder, it doesn't make sense to worry about the minor difference in wetted surface area between a moderate full keel or twin keels and a single fin keel. The turbulence from the prop and its aperture are going to hold your boat back far more than a few square feet of additional keel surface area, and their drag is going to increase with speed to a far greater extent than skin friction would. If performance under sail really matters above all else, trade your inboard for an outboard, and fair over the prop aperture, before you worry about your keel. On the other hand, if you are willing to accept the sailing performance disadvantages of an inboard in return for the definite advantages it offers, you should consider that twin keels can offer a similar set of tradeoffs.
Some Useful Twin Keel Links
Lord Riverdale's Twin Keel paper is the seminal work in this field.
Dr. John Letcher wrote an article entitled Why Twin Keels in which he makes an excellent case for them for cruising.
Nils Lucander wrote an interesting article about the wave cancellation effect of twin keels. You'll find it at these two links: Eyeing the Pump Effect page 1 and Eyeing the Pump Effect page 2
Patrick Bray has an article on twin keels. Of course, he quotes the Riverdale paper.