If you lack 'seishin', you differ no other than common people who can do similar tasks. All your techniques are useless if you don't have developed the 'Seishin' and you are not worthy of being called a ninja because all your techniques will then be for your personal purposes, desires and freewill.
As Momochi Sandayu says, "Ninjutsu is not something which should be used for personal desires. It is something which should be used when no other choice is available, for the sake of one's country, for the sake of one's lord, or to escape personal danger. If one deliberately uses it for the sake of personal desires, the techniques will indeed fail totally".
Even if it's about escaping personal danger to save your life, you should do it so only because saving your own life may help you save ten more. If you save your own life at the cost of leaving ten more in danger then that is not some thing worth of a true shinobi, and a samurai.
The practice of Seishin helps you be decisive and flexible in the most difficult times so you wouldn't have to think more on the situation. If you haven't kept your heart under the blade then you haven't fulfilled the true meaning of shinobi, which can be achieved by the use of Seishin and that's why it's so important. Not only in Shinobi traditions but the practice of Seishin first is the part of some Samurai Traditions too.