The Desperate Soul is not only quite readily available, it is also one of the most powerful tanks in the game. It is nearly immune to any reducible damage, and its health pool is not too low. It possesses a decent attack as well, making him an excellent fighter. Even if your enemy somehow kills it (without using Holy damage or removing its abilities), the Desperate Soul might just pop back into your hand.
In many bosses the Desperate Soul can be used to absorb all, or at least a most of the spells the boss uses. Other than the usual problems of ability removal and Holy damage, the Desperate Soul is also vulnerable to multi hit physical attacks, as its health pool is quite average.
On Awakening, this card gains Chained Soul, which cannot be blocked by Holy Damage and means this card will never go away until his abilities are removed.