A carry-on bag that fits under your seat is free.  I've done that a couple of times on short trips, and it was just fine.  And dirt cheap.  

And if you need to check a bag, just add on the bag fee in advance for something like 30 bucks (or something like that).  Still cheap at $85.  

Now, if you need all the extras and can't plan everything in advance, Frontier probably shouldn't be your choice.  That's fine.  But many other customers DO like to have the choice to not pay for many extras they don't need, and like to save money as a result. For some it may be the only way they can afford to fly.