Well I think the argument is that if you put a fairly shabby looking bike cover on it, no-one knows that there's a top of the range new bike under there without going to the effort to look. Therefore it's less likely to catch the casual thief's eye.
Ultimately, if a professional thief wants your bike & possessions then he'll get them whatever you do.
I may also use Pacsafe mesh with some of my soft luggage though, to make it slightly harder for someone who does look under the cover.
This is the approach I think I'm going to take with luggage for long-distance touring:
Tank bag that comes with me at all times for really valuable stuff - money, electronics, documents, etc.
Soft bags with pac safe for stuff I don't want to lose but wouldn't ruin my life if I did - camping gear, tools, spare parts, specialist riding clothes like heated vests
Soft bags for stuff that I don't care too much about losing - casual clothes, food
Then every bag will be taken off the bike at night to come into the hotel/tent with me.