Well, it all depends on the group composition. If you are choosing between Worm and TR, you are choosing between giving the whole group a 4% increase in Magicka 100% of the time or giving 1 player Minor Force every 10 seconds. The fact that they get a HoT is only important if they are taking a lot of damage. So it would depend on the Trial, the group, Warhorn uptime and other factors.
The HoT from TR is inconsequential. The reason I like TR is because of the Minor Force, but, again, it depends on the group and content. I definitely prefer TR over Worm, though--both benefit mostly magicka DDs (4% additional cost reduction won't really help healers, and tanks use Balance for magicka sustain), and I frankly would like getting Minor Force without trap (esp. since in certain content, I cannot get in to use trap) rather than a smidge of cost reduction.
I'm not sure why so many people look down on TR, esp. now that synergies are more reliable. Sure, it is a bit more situational than some of the other sets, but it's pretty useful. And even before the synergy fixes, we had our healer running TR for dungeon runs (because the group that I usually run 4-man content with have two ranged mag DDs).