It's a nice subtly for a series that's supposedly dark and cynical: The guys who are amoral (Tywin) and violate their own principles (Stannis) may seem to have the upper hand but eventually it blows up in their faces (Tywin suffering patricide, Stannis' usage of black magic ended up giving the Lannisters a huge advantage).
Stanis's problem is that he isn't using enough black magic. Aegon had his dragons and he used this for shock and awe.l He rode in like a pimp, incinerated his enemies, and made everyone bow down before him. And that's what Stanis should have been emulating.
Mel gives Stanis a decisive advantage that he should have flaunted from the very beginning instead of trying to keep it a secret. When he decides that he wants Mel to stay away from that assault of King's Landing because he doesn't want to be accused of winning the throne via black magic, he's making an incredibly stupid error. If I were in his position I'd want everyone to know that I won the throne via black magic. People do not fuck with sorcerer-kings.
And once the war is over Stanis would have plenty of time to get Mel to teach him her tricks, become a Lich, and rule Westeros forever.