The main question is if Mantra Siddhi is not defined then why so much of importance is placed on it.
1. For giving Deeksha of a particular Mantra to a disciple or seeker, the Guru should have attained the Mantra Siddhi of that Mantra. That included Purascharana. Mantra Siddhi is what qualifies one to be a Mantra Guru. If the guru does not have this qualification the mantra will not fructify.
2. Mantras give power. The use of this power is called Prayoga. Prayoga or application is a subject which is rarely dealt with in the open. The application or Prayoga has a number of rituals which are kept secret and passed on from Guru to Shishya.
The common prayogas are Snake Bite cure, Curing of other diseases like Jaundice, Dhana Prapthi and Parihara Pujas. The Abhichara Prayogas are Vasikarna, Sthambana, Spodana, Ucchadana and Marana.
Before performing Prayogas the Mantrik has to attain Mantra Siddhi after fulfilling all the requirements of Purascharana.
Some of the Mantras do not require the performance of Purascharana.
Mantra Siddhi is thus the prerequisite for becoming a Mantrik. It is the basic qualification.
There were times when Mantriks were powerful. Some of the communities in India have controlled wealth and were more powerful than even the Kings because of their knowledge of Mantras and their uses. Fortunately those days are long past and no one has any use for Mantriks now. Except for for some love sick youngsters who seek Vasikarana.
And there is no money in it. Most of the procedures have been forgotten and the manuscripts have turned to dust.
Of course we do see some Sadhus and Religious teachers performing elaborate rituals for Mantra Siddhi. They need this to become Tantriks, Astrologers, and Gurus.