Golf is a sport that does require flexibility. However, many times those seeking more flexibility may not have the fitness base to make flexibility really effective. Also, remember the old time greats who had wonderful and powerful swings -Hogan, Sneed, Trevino actually just had average flexibility.
So why the appeal with yoga for golf? First is every golfer looks for easy path to improvement. Often forgotten is that the tour pros exercise almost daily in the off-season, and during the season, they average at least three days per week in exercise outside of golf. So, their fitness base is really pretty good compared to amateurs, and any component they add will actually be effective because of their base work.
Another factor to consider is that in recent studies, older golfers loose distance because of the lack of spine rotation speed in the swing, not flexibility. This correlates well with the commentary on modern swings that tour players are actually using more compact swings to gain distance rather than through longer swings, the rationale for yoga and golf.
While yoga may help your flexibility, there is not a correlation, at least in the research with flexibility and distance and even overall performance in golf. In fact, a few years back one of the popular magazines followed a professional golfer who took up yoga. The result was he lost distance and eventually gave up yoga for more traditional forms of training.
Remember, while yoga may increase your flexibility, you still need a good fitness base of basic strength and aerobic conditioning before the yoga will do any good. Also, if distance is your interest, increase flexibility without increased spine rotation speed will likely produce no significant result.