There's no doubt that a quality hard shell guitar case is really going to protect your investment from damage. It won't be invincible in there (as we've seen some guitars break inside a case that was dropped) but it certainly doesn't hurt to have it.
Gig bags range from the thing that comes in a beginner guitar kit for $99 that isn't worth as much as the picks that were included all the way up to luxury soft cases that cost hundreds of dollars and/or fit multiple guitars. There are some quality pieces to be had somewhere in the middle, too.
If you've ever loaded your gear in for a performance, then you know what a hassle each large heavy object can be. A quality hard shell case can weigh more than the guitar inside. It always has a handle for one hand to carry it, and that means one arm is loaded down with just a guitar.
A good gig bag should have shoulder straps, and many times backpack straps. These can be invaluable when you load a couple guitars on your back, pick up an amp in one arm and another piece of gear in the other.
Now gig bags certainly won't protect your guitar if they get run over by the van... but neither will most hard cases. They give you a pretty safe way to transport easily and avoid bumps and bruises. Stacking them for long term storage may not be the best option. Hard cases win in this category.
If your guitar is acoustic and you're in an area where humidity is a concern (anywhere with less than 50% humidity) then a hard case helps to contain the humidity of your soundhole or case humidifier. A porous gig bag will likely leak out the humidity over time, allowing your guitar to dry out. A quality hard case will seal it in and preserve your guitar from the ravages of drying out.
So you may find that a hard case is best for long term use, but a gig bag can be helpful when it's more convenient. Just don't think your guitar is impervious to damage in any case... as we've seen the results to prove that wrong. Be sure your roadies don't throw an amp on top of your case.... and Gibson headstocks can break just from knocking a case over on the floor.