S/P is viable at the end game. You gain several abilities that greatly enhance survivability such as urgent heal, holy aura, and protection arrow. An excellent class in group pve play, as the priest general skills and most of the elites from s/p affect party members.
The problem is that a s/p does offer much, besides the sniper skill, to enhance DPS which is the ideal focus of a scout as the primary class. While sniper elite skill is nice, the fact is that there is still a 15 second cooldown attached to Snipe.
Bascially for S/P:
in PvE - viable in group play (i don't mean in instances, just general group play with quests)
in PvP - a mediocre combination since other classes can and will out DPS (and even out heal) you
Ultimately, main scout is dps, a /p is not the best way to go with scout.