Depends on the sniper's task, and how useful it is to remain unnoticed.
As noted above, Israeli forces use suppressed .22s for crowd control. This permits police to disable instigators without alarming the crowd - a guy in front just falls with a sharp pain in his leg, and few really notice anything other than less mob leadership. Carefully used, this is relatively non-lethal and calms the crowd.
Sub-sonic loads have the problem of shorter ranges. Since the bullet is travelling under 1000 fps, gravity does its thing and the bullet seems to fall faster, thus limiting its range. 100-yard performance is comparable to 500-yard standard (supersonic) performance.
Sub-sonic rounds have the advantage of being distinctly quieter, as they do not create the sonic "crack" caused by something moving faster than sound. Word is a subsonic .308 sounds like a .22 short being fired. Practically all the noise is limited to the muzzle blast, and even that can be tuned to drop off (but at the expense of bullet velocity).
If the target is rather close (under 150 yards) subsonic loads can do the job without much distracting noise; this is good for those of us who want a quiet shot but live in a no-suppressors state (like NY).
Using a silencer with a subsonic load can be very quiet indeed. Word is about all you hear is the firing pin drop and (if semiauto) the action cycle. Unfortunately, as subsonic loads are being used, distance is rather limited.
Using a silencer on supersonic loads has nifty effects. You get the standard distance (reach out and touch someone a thousand yards away), and with no muzzle blast the only sound is from the sonic crack...which is generated along the whole path of the bullet, and when the sound reflects off objects (walls, etc.) it is very hard to tell where the shot came from, despite the obvious fact that a shot occured. If you're shooting into a group, they know someone is shooting but are very confused about from where. Also, silencers tend to stabilize the barrel and the emergence of the bullet, thus improving accuracy a significant amount. One might use a suppressor just to improve accuracy.
Sabots may be used to fire bullets faster than normal, kicking a lighter bullet with the same force. .50BMG shooters use .30 caliber sabots to fire rounds at 4-5000 fps (very fast - any faster and bullets tend to disintegrate from the centrifigal force). This speed provides a flatter trajectory.
IIRC, firing sabots thru a silencer is a bad idea. The plastic holder falls off upon exiting the muzzle, and would tend to get caught inside the silencer and make a mess.
FWIW: silencers are tightly regulated, requiring BATF approval (fingerprints, local chief LEO signoff, background check), registration, and a $200 tax. Unlike machineguns or AOWs which require the BATF for permission to move or even take it out of state, the location of a silencer need not be constantly recorded. Note also that many states forbid silencers completely, and most that don't do prohibit their use in hunting.
). If the shot sounds like it came from in front of you your very badly wrong. If it sounds like that it came from the right or left and you stand a 50/50 chance of guessing which... that is unless you see the bullet strike.