From all of the test data I've seen and what little testing I have done and seen first hand, it would seem that the benefit of DTR on damage output of low potency DoTs is fairly small.
This isn't an accurate way of perceiving it. The benefit of any stat on a low potency attack is small compared to a high potency attack because higher potency amplifies the result of the stat change.

That DTR has a "small" effect on low potency DoTs is not surprising in the least. CRT has an equally small effect on low potency DoTs.

The real crux of this is whether or not the damage is actually being truncated per DoT per tick.

Even in the case that it is, DTR still has the same value as it has for all other classes -- except it has more awkward thresholds where you gain "0" value for a few points, then you gain the full accumulated value, etc. As a result it's more annoying to itemize for / deal with, but its general value is not diminished.

The only way a SMN is going to have meaningfully poorer returns from DTR is if other mechanics (such as the pet) result in DTR simply being ineffective or something (e.g. if pets had their own DTR, ignoring yours, which is unlikely).