If the best argument for a monster existing is that it may possibly represent a speed bump on your way to something actually threatening behind it, that is a bad monster.
In the example, if there was enough hellfire threat or if I was low enough on health, then you just slouch and kill the hellions. If not, you just walk towards the hellion until it moves around the red devil, kill it, and then the red devil. More practically speaking if I'm going to extended I'll typically have iron shot or some other ranged option just for red devils.
Edit: example action table behind the spoilers:
- Code:
Action | 1- 3 | 4- 6 | 7- 9 | 10-12 | 13-15 | 16-18 | 19-21 | 22-24 | 25-27 || total
-------------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------++-------
Melee: Unarmed | 63 | 148 | 185 | 326 | 834 | 820 | 859 | 774 | 4852 || 8861
Constrict | 19 | 41 | 53 | 59 | 64 | 135 | 151 | 77 | 738 || 1337
Tentacles | 15 | 35 | 50 | 122 | 111 | 257 | 297 | 239 | 1276 || 2402
Punch | 2 | 9 | 23 | 53 | 54 | 165 | 201 | 207 | 1081 || 1795
Fire: Blowgun | | | | 8 | | 97 | 8 | | || 113
Throw: Javelin | | | 8 | 6 | 4 | 2 | | | || 20
Throwing net | | | | 2 | | | | | || 2
Cast: Ice Form | | | | 4 | 48 | | | | || 52
Statue Form | | | | | 6 | 84 | 65 | 60 | 342 || 557
Lesser Beckoning | | | | | | | 2 | | 30 || 32
Regeneration | | | | | | | | | 123 || 123
Borgnjor's Vile C | | | | | | | | | 9 || 9
Apportation | | | | | | | | | 8 || 8
Aura of Abjuratio | | | | | | | | | 6 || 6
Shatter | | | | | | | | | 5 || 5
Deflect Missiles | | | | | | | | | 4 || 4
Iron Shot | | | | | | | | | 2 || 2
I'll give you one guess what I used iron shot twice all game for. Lesser beckoning may have been used on some red devils too but I also use that on real threats sometimes.