The main purpose of the i180 gear is to prepare for 3* crafts while avoiding grinding for favor items or the high cost of purchasing red scrip materials/favor items, but at the cost of slow progression. It's also a slow method of meeting the 2* requirements, but melded i150 gear is the better method, provided that you can afford the melds. If you can meet the minimum 2* craftsmanship/control requirements and have around 440 CP with food, you'll be able to handle 2* crafts fairly well.

The major downside to the i170 sets would be the time required to grind for favor items or cost to purchase materials, as well as the astronomical cost of melding them. From my experience, the cost of melding the i170 accessories alone is on par with all of the i150 melds combined.