Ok, I'll bite.

I prefer S turns to slips, but that's more about the glider I am flying today. When I say S turns, I mean two easy turns (back and forth) to make the final approach as long as possible. Not 5-10 sharp back and forth turns, for example. In my experience, an S turn is usually employed during a land out, and often when dealing with tall trees on the approach end of the field or a short field. S turns are an easy way to fine-tune the approach, without extreme glide angles, extreme changes in spoiler power or (low) airspeeds (hard slip, full spoiler). But S turns are very rare (for me) as it would be far easier (and safer) to do a simple circle at a consistent and higher airspeed, rather than a couple big, sharp S turns. So doing S turns (or slips) is very rare with the effectiveness of modern spoilers and the glide performance of even modest performance sailplanes.

All glider turns (at 'almost' all times) must be perfectly coordinated (any aircraft for that matter). Coordinated turns should be natural for the pilot, entirely effortless in fact. That's the key focus for the pilot during the execution of all turns, correct? Especially so when near that hard thing, otherwise known as the ground. So, if you are proficient with the basic skill of coordinated flight, you're fine with a mild couple S-turns on final. If you don't have the natural ability of proper turn coordination developed, risks are higher (especially when under pressure). That's a far broader problem than slips or S-turns.

So S-turns are very safe the pilot is properly trained and therefore flies the glider properly, at all times, without stress or difficulty. The only exceptions would be 1) slip landings - not very efficient (to nil) in a high-performance sailplane and therefore I never use slips when flying my personal glider. If attempted (I have tried slips a few times), they are very scary as the slip angle must be quite extreme to create any perceivable drag. -or- 2) utilizing a slight top rudder configuration while thermalling.

Slips work great in a 2-33 or a 126. But even a Grob 103 is fairly poor at slipping. An ASG29 or Lak17a (my experience) gets virtually zero benefits from slipping (my opinion) even in an extreme slip. And with full spoilers, flying relatively slow (60 mph) and at extreme slip angle is not what I want to be doing in any modern sailplane at 200-300 ft (again, my opinion). But in lower performance trainers, slips are perfectly practical and even fun.

Bottom line, turn coordination must be job one for all pilots at all times, from student pilots to advanced pilots. It must be natural and without exception, especially when under stress. When that skill breaks down, the risk increases dramatically at all stages of flight.

So as with all things, and with the question of slips or S-turns (IMO), it depends on the many circumstances involved.