I've had a look around and in the game, but all evidence I can find points to it just being an achievement, just like it is in lots of games. Seems like MW1 was just an exception.
I've referred to this for my source on necromancer skills: http://classic.battle.net/diablo2exp/skills/necromancer-curses.shtml
Lower Resist decreases an enemies elemental resistances and Decrepify decreases their physical resistance. Unfortunately, bone spells do magic damage, so I don't think either curse improves your bone spell damage (as Grace Note pointed out). They are only useful for your minions; Lower Resist helps your skeletal mages and Decrepify helps your skeletons. Also, if you are using Revive then those minions will do more damage when you use those curses (most monsters don't do magic damage).
Unless you totally focus on them and skills that synergize (see my sidenote), Bone Spear and Bone Spirit are ineffective because they do not scale up in Hell difficulty. However, Bone Prison and Bone Wall do. Also curses and revives scale up because they are based on the strength of monsters in Hell.
For a summoner, playing heavily to revive is a good plan. Look at some revive necromancer guides, this being a short example: http://diablo.incgamers.com/forums/showthread.php?t=707530. I recommend maxing Revive and Iron Golem, so that you have plenty of summons to work with and a golem for killing monsters to get that first revive. I also think you're better off replacing Bone Spear with Bone Prison and Iron Maiden. Then you can lock monsters in with your minions and have them damaged by both their own attacks and your minions, while boosting with other curses you have. Iron Maiden has the nice property that an increase in skill level increases the damage taken, not the spell radius like many other curses. Bone Spirit is appropriate to some extent for helping your Iron Golem take down monsters, but certainly not worth focus.
Sidenote:
To get an idea of how much damage a pure bone necromancer does, I'm in Hell difficulty with my necromancer right now using an all bone build, around level 76, roughly these skills (I can update values tonight when I check):
- Teeth: 10 (about +6 with items)
- Bone Spear: 20
- Bone Spirit: 20 (about +4 with items)
- Bone Wall: 10
- Bone Prison: 20
- Some other misc. skills
And I only do over 3000 damage per spirit. That's very little in Hell difficulty.
Best Answer
There is no reason to dupe in Diablo 2 on multiplayer LAN or open battlenet since the characters are saved on your local hard drive and you can download an diablo 2 character editor (there are many) to give yourself whatever you want, even change attributes on items to make them super overpowered.
As for closed battlenet, duping still exists, in particular, high end runes are still often duped on the ladder where they can be used in powerful ladder only rune words. The reason why runes are the choice for duping is because they cannot 'poof' if they are immediately used in an item for a runeword as they take on the UID of the item they are put in, but if two duped items with the same UID enters a game, they will dissapear.