Weapon Mod Oversight?

I always wondered why weapon mods are so limited. I mean, where are mods like:
Increased echo duration
Increased shout duration
Increased weapon spell duration
Increased spirit duration
Increased hex duration
I know there's more than the above, but these are the ones off the tip of my tongue. Is this a 255 bit code limitation, or due to most mods hardcoded before factions/nf/eotn?

mainly because those mods would be rather overpowered with their respective builds... especially the hex duration one.

Those skill types are balanced to not have +20% mods used with them, therefore, if those were added, Anet would have to take 20% off the existing duration to rebalance.

Doesn't really matter. A real mod oversight is the lack of +20% damage mods (vs species) for Daggers, Spears, and Scythes. /want +20% dwarfslaying for daggers, for dwarven boxing ;(

All of the things you listed can be extended by available skills.