I'm not going to repeat Elmacca here. But add something to his responce.
4. Higher potentials are ment to be used for all around players, they do not hit their cap that fast thus will not make training secondarys that hard to improve (when some potentials would be at their cap already). I don't think high potentials were ment to be used on training few skills to 20 -> 20+.
5. I guess it's not ment
buy a high potential players for a cup final
it's high salary players. How many of the manager have successfully done this. I've seen one manager trying this for promotion. Failed miserably, why -> those players are hard to move, paying salary even once will eat away your pressious funds. It's a big risk. If you win, then by all means the risk was worth it. If you lose, you will lose a lot. The tradeoff makes this kind of behaviour ok.