I give them $5.
The "interesting" thing is that the porters are all part of the Longshoreman's Union and many do the baggage handling for extra money.
The average wage for a Longshoreman is around $69,000 a year. This is according to U.S. Department of Labor. http://www.dol.gov/owcp/dlhwc/NAWWinfo.htm
The cruise line employees are not allowed to touch your luggage until it is on board the vessel.
I really don't like tipping these people but do so out of custom and an irrational, or maybe rational, fear that my luggage will end up in the water or sitting at curb if I don't.
I agree that tipping can seem to get a little out of hand.