The break even is on the cost to earn the premium.  For every $1 in spending over $10K you are earning 2x what you would on the regular no fee card. Obviously if you don't spend more than $10k there would be no point.  But since most people will easily spend more than $1,000 in monthly expenses that can be charged to a credit card the fee is worth paying.

So even if you have more than one card you can often still benefit from premium cards.  We have 2 no fee cards and a premium card - the premium card gets 90% of the transactions.  The no fee cards are simply for convenience.