I was going to tag this on as a reply to Steve's "Stoker Bar" thread but decided I'd just take the whole topic to a slightly broader level.....
Cowhorn bars -- as best as I can tell -- only serve one real purpose; they lower the cost for producing a fully assembled tandem by a few bucks. Cowhorn bars are less expensive than drop bars and they eliminate the cost and labor associated with providing and attaching stoker pegs. The latter may have helped with keeping the cost of new tandems in check OR freeing up a little margin for the producer; I'm not exactly sure which was the real motivation or if there is even a distinction to be made. Cowhorns also reduce the weight of a tandem by a few grams -- also a marketing advantage -- which may or may not be more important than giving your stoker more hand position options depending on how you use your tandem (e.g., for racing, pain and discomfort is part of the game) or how concerned you are for your stoker's comfort (see additional comments for tandem captains regarding Stoker Comfort, below).
So, just when did cowhorns become the default bar for stokers? A certain major producer of high-end tandems introduced the cowhorns back in the late 90's with the usual dubious techno-babble that accompanies each aspect of their product's features. Being the lead-dog in tandem producers (which admittedly is a position they earned and deserve), the major new-tandem buyer demographic (new empty nesters, one of who had been or was an active cyclist) bought into the latest "trend which, in turn, caused some of their competitors to follow suit so as not to lose a sale because they didn't offer the same features (and, well, they did reduce costs). Thus, a lot of stokers were put on tandems with handlebar "horns" that were way too wide for them @ 47cm -- not practical nor comfortable to use -- which relegated them to the flat section and a single, useable hand position. For a large number of teams, this arrangement "seems" to be quite fine. However, many astute teams have, instead, spent the $100 or so to replace the cowhorns with dropbars/stoker pegs/new bar tape and, thankfully, some of their competitors never did buy-in to the new trend and still sell their high performance road tandems with proper drop-bars and stoker pegs.
Bonus Material For Tandem Captains:
IMHO, most stokers who never rode a bicycle as an adult -- up and to the acquisition of a tandem -- don't know if they are comfortable or not because, lets face it, upright bicycles are not inherently "comfortable" machines.
Therefore, many stokers merely learn to accept and sometimes even adapt to improper riding positions because they assume "that just the way it is" because no one ever took the time to work with them on their riding position and they're reluctant to complain. Well, that's not true. Many stokers ride in discomfort which can lead to "I'd just rather not ride today" or "honey, we need to stop again" or "honey, my _____ hurts when we ride" which is always a good indication that something isn't quite right: all of the foregoing can lead to an under or un-used tandem.
Therefore, to all captains, PLEASE be sure that your stoker gets a proper bike fitting by a professional bike fitter, e.g., someone certified in the use of the "Fit Kit" or a Serotta Size Cycle either before you go shopping for your first tandem shopping, most certainly before having a custom frame made, and without question at the first sign of a fitting problem.
As to whether or not the stoker's "optimum" personal bike riding position should be replicated on the tandem the answer is rather an easy one: just figure out how sub-optimum it will be and ask yourself if you'd be willing to ride thousands of miles over the next several years on a bike that was as proportionately mis-sized vs. your optimum riding position.