I agree - it depends who you cruise with. We usually go with P & O (or RCCL) and apart from the formal evenings, I just take my usual casual/bit dressy clothes. On a 2 week cruise you may need 4 formal outfits which could prove pricey if you have to buy them all at once! I've now got a wardrobe of gorgeous evening wear courtesy of ebay, and at very little cost!
Yes they are. You need to know which cruiseline you are going to book with. I suggest you look at the brochures and decide which best suits you. Some lines have formal dining and some don't.