Malphus has comparatively lower health and damage, as is typical of a looting monster, which means that it will probably not be used much in attacks, let alone defence. Though it is arguably quite strong for a looter, with 450 health and 100 damage at the very beginning, It still does not compare to other monsters, especially since by the time you can get Malphus from Inferno, your Overworld Yard should have access to stronger monsters, and in Inferno, any Zagnoid and Spurtz combo would destroy your Malphuses. Malphuses CAN be used reliably in the earlier stages, to quickly steal loot after Defensive Tower monsters have wrecked the towers, and its ability to leap over walls could be an asset to your attack, but shortly after leaving the earlier stages, after you get Valgos, Walls are absolutely nothing anymore, and other monsters can benefit from wall-less yard after Valgos is done, not just Malphus.
My rating of Malphus: not a commonly used monster, and is overshadowed by other monsters.
By the time you get Malphus, you should have Brain, or Fang, or some other better monsters that kill Malphuses with ease. You might even have Zafreetis, who will never catch up with your Malphuses, making them easy-kill targets for the higher-leved towers following a Level 5 Town Hall. Your enemy might even have bunkers filled with killers to get rid of your powerful monsters, let alone Malphus! You wouldn't use them to attack anything... after all, even one hit from a level 5 Sniper kills a level 1 Malphus, and a level 5 Sniper Tower is not a rare sight.
You can consider sweeping away the towers of a base with Zagnoids, and then send the Malphuses in to steal the loot. Their leaping ability makes them pretty efficient looters, going from RG to RG with ease. But once you get Valgos, you won't use Malphus anymore - Valgos' AI to eat through blocks will help any one of your monsters get to the enemy buildings quickly. Also, any Zagnoid and Spurtz pair from the Compound will wipe out a Malphus. And then once you get Grokus...who needs Malphus? Unless you're low on resources, it's unlikely you'll ever use Malphus again.
By the time you get Malphus you'll have Fangs, Banditos, Eye-ras...what on earth could a Malphus do against a Champion with a Zafreeti behind it?
In the earlier stages you could use them with other Zagnoids and Spurtzes in your Compound before you used them in an attack...but they might get themselves killed first by running out so fast with their fragile bodies. At least they protect your Yard from the continuous lure baiting technique, as they quickly reach weak monsters and finish them.
But once again...once you get Valgos you have a far more efficient counter to the continuous lure technique, and a more durable fighter that kills Malphuses and Zagnoids and whatnot faster at that lower level.