Without poison dagger all durations of all poison sources averages (there are exceptions - like charms apparently, which don't appear to count). So if you have a really short duration on an item it can reduce the overall duration. All poison damage rates add up, however, so more is always good in that respect. When you use poison dagger, the skill duration doesn't average like the items. Instead it adds on directly to that averaged duration from all the items. Unless you only want to have to stab a boss every 20 seconds instead of every 12-14, it's really not an issue IMO. If typical monsters aren't dying within the dagger skill's duration you don't have a very strong skill on your hands.
Kitriara,
Even if P-Dagger could spawn on heads Trang's Wing would still trump any magic or rare head you could find anyway since with two other Trang items it will give you -25 poison resistance. If one of those items is Trang's Gloves (+25% damage) it's like having 5 free perfect poison facets. If you're looking for something to use in the meantime just keep your eyes open for PNB heads and let people know in the trade forums that you're looking for one. Too many people ignore magic and rare heads. They could really be quite useful with good mods. Trang's gloves, shield (wing), and armor aren't that hard to find and will likely trade for PGems later in the ladder season. The Belt (which is quite nice) and Helmet (which is crap) are considerably harder to find and worth a lot more.
