Sephara, Sky's Blade is a creature who is able to reliably cut her own mana cost down to a single white mana, by simply controlling a substantial number of creatures with flying
Jumpstart doesnt have that luxury
However, since very few exist and they provide intriguing and powerful effects, a number from Mirage have retained high price points, this one included
Discarding your entire hand and only drawing three may not seem like a good idea if you have seven cards in your hand, but if your hands empty, then theres hardly a downside at all
You can tap any untapped artifact you control to pay the cost of the mana ability, including an artifact creature you haven't controlled continuously since the beginning of your most recent turn
You can cast flash cards during your main phases , sure