There is really no point in EV'ing a Pokemon for Defense unless they already have the defense to pull it off. You're usually better off putting the EV's in HP if you are trying to make them more bulky.
In general, you should never give Gengar any form of defensive EV's, since he has a shallow Defense and Special Defense and couldn't make use of those EV's.
As for other setups; you have the common Wish + Protect set up. It guarantees a two turn heal of 50% Health restored and can be used to heal your other Pokemon. Another common tactic is to have Stealth Rock, Spikes, and/or Toxic Spikes on the field and have someone that can abuse switching, through either Roar, Status inducers such as Yawn, or just by their presence. (Blissey.)
Really, though, the Metagame is always evolving. Things that worked flawlessly last month may not even be viable anymore. That's an extreme scenario, but I've seen it happen. A good example of this is Celebi; when Scizor first rose to power, it was led to believe that Celebi was free set up bait for a free U-Turn as it couldn't do anything about it. Now, however, almost every non-defensive Celebi, and even some defensive ones, are packing HP Fire to obliterate the metal bug.
Hope that helps. --- "See what I did right there? Oh, wait. This is the internet. C wut I did rite thur?" HG Friend: 3953 9198 8725
But it seems like it would be pretty stupid to make a team consisting of entirely sweepers that lack any high defensive ability. No. Teams of 6 sweepers are GREAT. Teams of 6 walls can be GREAT. Teams with a bit of each are generally much less threatening and more luck based (requires your team to have just the right counters for the opponent's), but can be good if you center the team around one main sweeper or game winning gimmick.
In my own head I was thinking a Gengar with Ev (Attack and Spc Def). Gengar's got terrible defenses and really shouldn't ever be taking strong Special hits. The hits he's going to take should be physical ones, like Bullet Punch or Pursuit - but EV'ing in Defense does not really help him unless it's for a VERY specific reason. SpA and Speed are the way to go, with Gengar.
Or an Butterfree status inflicter, Ev (Def and Spc Def) or (Spc Def and Speed) Since we're talking about competitive play - don't use Butterfree. It's got terrible stats in just about everything and it's only real gimmick is done better by Breloom, who is awesome. For casual play, Butterfree should probably go with max Speed so it can Sleep Powder or U-turn before the opponent kills it.
This site has good movesets and EV spreads for all pokemon: http://www.smogon.com/dp/pokemon/
Don't use all sweeper. Generally using 2 physical sweepers, 1 special sweeper, 1 wall, 1 sponge, and 1 miscilaneous is a good balance. That is very newbie-ish advice. Building teams like that is a good way to hit a wall in competitive play. You'll have a hard time beating players that know how to build good teams, without resorting to some odd template.
This topic explains some of the basics of team building, but also assumes you know a bit about the metagame: http://www.gamefaqs.com/boards/genmessage.php?board=925601&topic=48198778 --- If you are currently in an argument with me, consider the following: http://www.qwantz.com/comics/comic2-1673.png