As a Dekaron Dil player , Do you know why only use poison not use other element ? I found this article from other site , Credits to k4kashiiii , No more talk , Let's begin :

1) As you know , there are physical and any element will add damage to the Twinblade skills , Poison sword adds 14% poison damage is better to have poison gems to get like 29%~ poison damage . When your casting staff skills and switching to gain the magic damage from your TB , As all staff skills poison element will add more damage .

2) As for bagi's using poison element . One reason the other physical based classes uses poison element on their weapons .

Resisted by any other class , Poison is the least . Mages have high resist to light , ice and fire . curse and poison is least, but there are a lot of curse resist mobs .

At last , As a TB summoner use poison gems is to be able to cast hellburst orincrease poison than switch to your TB's since it will have higher magic damage and +15% poison damage .

You can do it with palpus but the only hit that gets the bonus is the last one , this is the reason the smart high level summoners use poison gems , In addition , They make weapons look good , ice's glow just isn't visible enough .