This card has an important place in my travel strategy. I keep this card along with other miles earning cards.
Even when you have miles, you have to pay some pesky fees: award taxes, resort fees. This card helps you get those costs in control
Once in a while you need a revenue flight to complete your trip. Sometimes there are good fare sales as well. This card is perfect for that.
Sign ups are awesome but you need a good quantiyt of miles to get most of that. e.g. you got 50k us air miles from the sign up, that doesn't really get you anywhere. But if you want to do a nice trip you can buy the miles in their share miles promo (once in a while). The cash from this bonus comes in handy then.
I don't currently ahve a credit history that can handle 4 cards every 3 months. I will probably get there in a few years. Till then I will have to make do with MS