Yea the problem is that a 999 attack return barely misses the 2hko on skarm (nor sure about this, i'm on phone and can't check the calcs atm), meaning you are able to curse up to +6, but in the long run you can't prevent skarm from coming in every time and try to phaze you out, get damage on your sub or use toxic, or at least burn a big part of your pp.
Swampert is usually used for its defensive abilities. This one however fits physical offensive team pretty well. Under sandstorm, it can kill (or even weaken) one or two Pokemon (depending on the situation/how much your opponent fucked up) and those are usually troublesome to deal with for offense (Bulky Water/Snorlax). Torrent boosted Surf/Hydro Pump hits hard. Its natural bulk is useful to take one hit or two against offensive team from the like of Metagross, Tyranitar or Salamence but don't overrate it, the lack of defensive EVs really makes it frail and it won't switch-in more than once (or maybe twice).
As for the downsides, it has some hard counters like Starmie, Roar Cune (if it comes when you're full health) and Dugtrio is still faster even at +1.
Umbreon @ Taunt/Mean Look/Baton Pass/Wish
One of the reyscarface favorite: TrapPass Umbreon. There is very little you can do about it when you run an (and they are often) unprepared defensve team. A bit underwhelming against Offense. Can 6-0 really easily with the right partners (hi DD Taunt Ttar).
Sub/CM/Roar Suicune (with max HP and enough speed to outRoar the other Cune at the very least) is really strong and win games by itself when pulled off at the right time.
ADV Hariyama deserves mention IMO. Great TTar check (eats specialtar for breakfast), nice offensive prowess, decent bulk (although we wish it could be better). Two viable abilities in Thick Fat and Guts. Good user of Restalk because of them. Gets STAB Cross Chop, and can also make good use of Revenge if you hate the PP (Lax becomes more of a problem though). Generally I find myself using it as an impromptu part of a bulky core because there are times where simply nothing else fits- has a Rock resist which is crucial to being able to take physical hits in ADV, in addition to Bug, as well as Fire and Ice resists provided by Thick Fat. I would never build a team around him but there are teams that I find only he can fit into, and he pulls his weight.
I sort of preferred skuntank over drapion since it made for a pretty decent wall breaker if you use it for its strengths over drapion. Its taunt + sucker punch was pretty cool for stopping walls cold while nailing faster psychic pokemon. Its pursuit would also keep frail psychic types from switching out so they are more or less doomed. It can be a fairly decent mixed attacker with fire blast to get passed physically defensive steel types like aggron and steelix and it could go boom on bulkier walls. Finally, it could make fat subs with its nice hp which keeps seismic toss from breaking them in one turn and forcing pokemon to eat a sucker punch to break its subs. Its aftermath is nice too for causing rocoil and potentially finish off severely weakened pokemon. Its somewhat difficult to use in ou though since it cant do anything to heatran besides maybe keep it from setting up rocks.
with gen 5 on the scene, its now time to revive this thread in order to show off the niches of pokemon in the weather filled gen and other gens if some nihces were missed. Heres one.
It can be a pretty decent spinner under the rain with its rain dish ability. Its able to tank a decent amount and burn physical threats with rain boosted scald spam. It can shuffle pokemon by using roar/d tail. You can almost call it a better donphan but the thing that keeps this from being true is that it lacks stealth rocks. Also that its prone to thunder from the rain thats helping it so much. Finally, pokemon who doesnt care about status such as celebi has no problem switching in and setting up since blastoise does not want to get nailed by its giga drain/leaf storm. Its also prone to status itself so it probably would rather not spend time trying to use foresight on a jellicent.