Try to compare another summoning card - the new General's Army. This card still summons a good combination of army (even after the change) plus we essentially pays nothing to summon them (only the initial cost is raised). What's more we actually have a net gain - a generous +15 recruits - by playing this card. The combination is fixed so it's guaranteed that we must get what we want.
Dragon egg, on the other hand, cost much to play. And we still have a small chance summoning a cheap common/uncommon card. At best, we may get a good rare dragon. due to the random nature, sometimes we may get a sub-optimal rare dragon which we don't want. (OK! It's uncommon. But does it really justify such a bad treatment?)
What a difference! What a bias! :-P
Unless you are so desperate to get a rare dragon (at little risk), Scepter of Summoning is a better card. This one is a card for rubbish bin.
The author should at least guarantee a rare dragon. ;)