Lash of Pain is a 20 yard range, not entirely melee, no, but not entirely ranged either.

Firebolt has a 40 yard range.

In any situation wherein a boss moves outside of the range of Lash of Pain, the Imp now becomes a DPS increase over the Succubus. Anytime a boss moves outside of the 7 yard melee range, which is the range of the Felhunter and Felguard, then Imp becomes a DPS increase.

Demon Soul-

Felhunter - increases periodic shadow damage by 20% for 20 seconds.
Succubus - increases Shadow Bolt damage by 10% for 20 seconds.

For affliction, there simply isn't a comparison here. Even without being in the Drain Soul portion of an encounter, using the Demon Soul Felhunter and switching to a Drain Life filler over Shadow Bolt would yield higher DPS than staying with the traditional Shadow Bolt filler. The only reason that it isn't used now is because using the Felhunter is a DPS loss over the Succubus. Once the patch is released, this will no longer be true.

Even now, without the patch, it is higher DPS to save two Soul Shards for the end of the encounter. One is used to Soulburn a Felhunter whom you then Demon Soul, recast DoTs, and channel Drain Soul. Once Soulburn is up again, you switch back to the Succubus.

Demon Soul Succubus is not designed for affliction; and it's a poor argument to try and say that it is. It's only a solace that affliction can make use of the ability for a fraction of their rotation because using the Succubus with the Lash of Pain Glyph is the highest DPS. Without that, it's actually rather useless.