Doesn't it unnecessarily advertise your presence and indicate your vulnerability to every passing thief? The answer is, surely, no more than a suitcase. The main difference between them is that whereas backpacks are associated with budget travellers who are going to be walking to their hotel, suitcase are associated with those who can well afford a taxi. Unless you can take the taxi as well as the suitcase, carrying the latter will not make you any less inviting a target. In fact if anything it probably increases your chances of being robbed. It is easier to snatch a suitcase from your hand than to run off with a backpack attached to your back.
Source: Handbook for Women Travellers, Maggie and Gemma Moss

Watch Your Daypack...

Whether in the airport or in a restaurant, I never leave my pack pack out of my sight. If it's on the floor under my chair, I put my foot through one of the straps. The least tug on it and I know immediately. If it's beside me, I pile my coat, etc. on top so you can't get at it easily. I keep my valuables in a zippered compartment in the back inside section of the pack -- the part closest to my back. I pack thicker things -- a small towel, my sweatshirt, etc. at the front of the backback -- the part facing out and most vulnerable. If a thief tries to slit my pack with a sharp utensil -- he can't grab and run, unless he covets my very well-worn sweatshirt.
Source: Evelyn Hannon, Editor Journeywoman.com