If you look at the options available to the scout, there are two observations that can be made.
1) The weapons are specialized by type. Shotguns are okay, but without a AP it loses any hope against heavier types. They are further limited by range and with only two dice per, they still aren't wiping the floor of light inf. Snipers are great against heavy inf and monstrous creats, but it is sigle shot.
2) They have only a 4+ armor sv. This means that in the assault they have little suitability to sustained engagements, especially against assault specialists.
Putting these together, we see that scouts want to hang back and camp. Snipes are the best option for this purpose. The idea that you are going to blitz with scouts into an assault is suicide and something better can be bought (dread w/ drop). If you want to kill blob infantry, scouts don't have the dice load and there are better options (thunderfire cans, devs with plas or hvy bolts). That really leaves one major target, heavy infantry and monstrous creats.
Now, I have seen an argument for scouts as squatters on obj. Here is my take. Bikes have more maneuverability and sustainability with higher T and Armor sv. Meaning, you can use the bike for more than just squatters, then rush to an obj for VPs. So only get the scouts if you want to ensure obj sec and you have little points to spare.
As for the choice on spec weapons, I was divided for a long time. MLs have the generality that scouts are missing in their main choices, plus they can disrupt vehicles. Yet, generalist isn't what the scout is on the field for. He is there as a specialist, and if you have taken my advise a heavy inf spec. To this end, hellfire ammo is perfect. Blast poison 2+ is king.
So, are snipes better than grav Tacs against heavy inf? Let's test it!
1 term sqd with a cyclone miss is 200 pts.
Scouts can get two squads of 5 snipes, one hellF H. bolter with 8 pts to spare.
Tacs can do 7 botlers, 1combi-gav, 1 grav, and 1 grav can at exactly 200.
I tested it and the Tacs succeed in killing the Termi sqd roughly 1 in 3 engagements with heavy casualties. The gravs are just too short for range. Even if you drop the cannon and get a rhino, the cyclone miss still reeks havoc for the Tacs. The snipers on the other hand win roughly 7/10 engagements and do so usually with minimal casualties. They can shoot the terms with more dice at 36" then the terms can dish back. Even one snipe sqd can obliterate a term sqd in 3-5 volleys, and have even done in two with hot dice.
Finally, one of the posters said that scouts cant hang tough in big matches, and he is partially right. No army can obliterate 1500pts of enemy models in a single turn. As such, by putting the scouts forward in such a scenario, you are asking to be assaulted and killed. If you are going to use them, You got to put them in the back.