When you talk about SM buffs you need to look at knight/IB (the classes competing for the tank role slot w/ SM) and see why they are preferred or even mandatory in a 2 tank or solo tank setup.
Looking at IB the most obvious thing it brings that other tanks do not is punishing knock. Because of how well it counters DoK's and how much of an impact 50% less healing is even if it is just kept on 1 healer this skill alone is reason enough to disregard anything SM can do.
Knights are well... knights all I have to say is "encouraged aim, dirty tricks, focused mending"
What unique facet do SM's bring to a group? Whispering winds is about all I can think of, which would actually be very good but destro does not have shatter limbs which means you aren't really getting much out of it aside from some things which I don't really believe are necessary, such as WP being able to spam group cleanse, or WL being able to spam lions fury. Reduced cooldowns are nice, but not near as strong as punishing knock, or knight 20% crit 15% healing etc... Not to mention WW is a 13 point ability which means you are pigeon holed into a spec unlike IB/Knight that get their "good" abilities not even half way up the tree giving them tons of flexibility in how they build.
The AoE nature of their punt can often times be a hindrance more than a blessing thanks to CC immunities... another skill that was nerfed far more than others by that patch that was never changed to reflect what happened.
Unless the meta drastically changes SM really need something, and that something needs to either reflect what is most important in this game at the highest level. And that important thing can basically be boiled down to burst / overcoming healing and inversely protecting your group from burst and making sure your healing isn't overcome. Which is why the knight is considered the best 6 man tank in the game. The 20% crit for the team is gigantic for burst, the 15% more healing for everyone is the same thing. Then you got stuff like banish darkness which is amazing for removing guard, or throwing anyone into a position they don't want to be in.
IDK what SM should be defined by, but stat steal that doesn't stack with anything that is already covered by many classes often times in better fashion, and whispering winds is never going to cut it when you are facing destro premades who are going to be running the best comps they have available to them.
In this game unless your groups skill is on a whole other level than the group you are fighting you have to take advantage of the best classes you have available to you to compete, and right now I just don't see why anyone would be running a SM over IB/Knight in that situation.
No class should ever be mandatory, there should be a plethora of viable comps that benefit from running a different solo tank / set of tanks. But right now I feel setups like say knight/IB/WP/RP/Slayerx2 just overshadow everything on order. RDPS I think are going to get a reality check in T4 because right now in T3 they are as good as they are going to get. Hopefully changes to classes and the overall meta will bring in more viable comp variety and allow the SM to settle somewhere nicely with some changes.
Spoiler:
Inspite their weakness they do have some goodies to give to group and you can spec it through the mastery trees to be usefull to any kind of setup of premade I dare say.
