I flew Frontier yesterday (for the first time!) and was thinking the exact same thing. $30 is the same price and similar benefits as the legacy airlines for buying up to regular economy from basic, but starting at a much lower base price. I was very willing to pay the extra $30 on a $97 ticket vs. on the $199 Alaska was charging (kind of last minute).
Overall, I was pleasantly surprised by Frontier, and if it's a trip where I can tolerate lower operational reliability and the sparser schedule works, I'd be happy to choose Frontier over Alaska or other legacies again.
When things go well -- which is the majority of the time -- flying Frontier is fine (assuming you can live with the tight legroom on the aircraft). But when things go wrong -- as they do with some regularity -- flying Frontier can be rather awful, with bad customer service and recovery. I would discourage people from booking connecting flights -- the risk is simply too great for the reward in most cases -- but for nonstops, when the price is less than half for "the same thing" -- it can be worth the risk if you value saving money over comfort and convenience. With my status match, I pay an average of $30 per flight, sit in the emergency exit row (or the new guaranteed empty middle seats) with free baggage and that works for me. At $100 a flight, I wouldn't fly them.