Putrid Imp is one of the few cards in Dredge that can fulfill one of three roles (warm body for DR/attacking, discard outlet,
Ichorid fodder) whenever necessary (it's worth noting that
Phantasmagorian is able to fulfill two).
I might be wrong here, but Putrid Imp has that distinction over other card choices (I'd be interested to know if there's any other card that can fit that criteria).
I understand what you mean by "interaction with counterspells being unnecessary", but I'd like to point out that running
Putrid Imp doesn't require you to interact "unnecessarily" - you can always opt to go with the DDD route if it's absolutely necessary. In comparison, running Phant (and not running other discard outlets that will require you to
interact) does not provide you with the option to deviate from the DDD route, which makes the casual
Thoughtseize or
Inquisition of Kozilek affect you like a Timewalk (and
Hymn to Tourach might even become two Timewalks! :laugh:).
In summary, the weakness(es) of
Putrid Imp won't require you to do something
unnecessary if the situation calls for it, and in comparison,
Phantasmagorian's weakness will always require you to helplessly
"interact". In that sense, it's
Phantasmagorian that's doing the unnecessary interaction, rather than
Putrid Imp.
As a side note, it's worth noting that counterspells are more abundant in the Legacy metagame when compared to discard effects, but I believe that that consideration will require a separate discussion.
Kind Regards,
jares