We have always tip anyone who handles our luggage. When we arrive at WDW resorts and we have Magical Express pick up our luggage, we leave an envelope with tip and note in our room for bell services when they bring the luggage. They've always taken it. When bell services picks up our luggage for the cruise, we tip about $2/luggage. When we take the
DCL transfer to the port, we don't tip the driver. When we arrive on the cruise and know that our stateroom host brings our luggage to our door and pick up our luggage the night before we disembark, we leave an extra tip of about $2-$4 per luggage in the gratuity envelope.
The first DCL cruise, we didn't realize that it was the stateroom host that was handling our luggage until the night before we disembark, so my husband found him and handed him extra tip. He was surprised and appreciative. I'm not sure if everyone knows that they handle the luggage as well.
When we disembark from the ship, we pick up our own luggage...we don't use porters.
When we take Magical Express to the airport, we usually take our luggage. So, when the driver takes the luggage out at arrival, we tip him then (about $1/luggage). If we use the Resort Airline service at the resort, we bring our luggage to the counter and once they tag and take them, we give them a tip (about $1/luggage). In this case, we don't tip the driver when we take the ME to the airport.
I do understand why some don't tip. I don't think they know to do it. I don't think they're being cheap. My inlaws joined us for our Alaskan cruise and when luggage service at the Pan Pacific Hotel in Vancouver came to collect our luggage to bring to the ship, they didn't tip. We noticed this and tip for all of us. When we mentioned it to them (casually of course), they didn't think they had to tip. We just explained to them about tipping when anyone handles their luggage.