If you are researching this card, then one of the things you have to be aware of is that this is not the only card where you can earn miles for Frontier Airlines. It turns out that Frontier is also a partner of the Membership Rewards program. That means that Amex charge card holders can transfer points (MR points) to Frontier miles. The ratio of transfer is one to one. Below is a screen shot showing the Frontier page on Membership Rewards.
The screen shot above shows the the page where you can transfer your membership reward points to your Frontier account. You will notice that if you do not yet have a Frontier account, there is a link that takes you to the page to sign up. You then have to link up the account with Membership Rewards.
There is also a function on MR's Frontier page where you can search for your flight and figure out how many MR points you need to transfer over.
Transferring MR points over to Frontier is not the only great thing about American Express. The other thing to note is that they have different cards with different points earning formula that might be better suited to you. For example, they have the Premier Rewards Gold Card (See review here), which allows you to earn triple points when you book an airline ticket with the card. You can also earn double points on US stand-alone gasoline stations. Therefore, you can technically earn more "Frontier miles" with this Amex card than you can with the Frontier card (bear in mind that the Premier Rewards has a $175 annual fee).
This is an alternative option that you seriously have to consider because the Frontier MasterCard does not offer any perks like free checked baggage or priority boarding. Transfer from MR to your Frontier account is also almost instantaneous so you can do last minute transfers if you need to.