RE: soft luggage packs.

You do not need a roof rack, but one helps.

Yes they can wear away the paint! you can put down a fleece blanket or old sheet on the roof to protect the paint.

A soft pack can be folded and put away if you have no place to put a hard 'clam shell' carrier.

The hard clam shell carriers can be locked and are more aerodynamic that a softie-but with a soft bag you can snug it down it you aren't using all the space in it!

And the cost is something you may want to look at....will it pay for itself after a few uses???

Yakima and Thule make 'systems' that are used to carry stuff on car roofs.

It's been awhile since I looked at their websites tho......Good LUck!