I usually buy the three day pass in Venice without much calculation or concern. The flexibility it provides makes it worth the expense. And it seems to me that you've answered your own question.

Yes, each person needs their own ticket or pass. You are also correct to assume that the only way to consider the value is to compute the number of trips you anticipate.

Depending on when you plan to travel, you might find discounted passes at VeniceConnected. The discounts are available based on advanced purchase and travel season.

http://www.veniceconnected.com/

You need 5 trips to break even and your itinerary shows at least 5 trips. Of course each passenger needs either a pass or a ticket. You also need a ticket for your baggage.

You do NOT need a ticket for your baggage. Go here for info on the passes: http://www.actv.it/en/movinginvenice/movinginvenice

> The discounts are available based on advanced purchase and travel season.

http://www.veniceconnected.com/

I have bought pass ( 72 hrs ) on Veniceconnected with a little discount ( blue zone - Standard online fee ). However they seem to have a problem since some time now. I read it first on Tripadvisor Venice Forum. If you check their price calendar now, until the end of Oct, you see all the days in blue - Standard online fee ). Normally it should show both blue and green - reduced online fee/the lowest rates. And from Nov 1st, every day is green ( the lowest). However if you click further to see the prices, either blue, green or red ( last minute - no reduction), you see the same non reduced rates. At least that's how I see it right now.

Seattle Michael is right. Luggage is included in the passeger's ticket ( 1 piece of total 150 cm - offcially).

Passes are also helpful since, as I discovered on my visit last year, not all of the vaporetto stops have ticket machines. It's very handy to just get on and not to have to worry about purchasing and validating individual tickets each time.

We were in Venice in late April/early May this year. We bought 72 hour passes and used them many times. It made planning easier as we didn't need to think about how to get from point "A" to point "B". When we used up the 72 hours, I wished that they had offered a 96-hour option.

As others have said, if you expect to make at least 5 vaporetto trips a 72 hour pass is worthwhile at E35 (as opposed to E7 for a single). One per person. However, if you only have 72 hours or less, unless you are staying somewhere out of the way you may not need to use the vaporetto much or at all - everywhere or almost everywhere you are likely to go is easily walkable. When we used to come on holiday here it could be 3 days before we used a vaporetto at all.

kappa1, re your not seeing discounted tickets online. I don't know if/when this takes effect, but it was recently reported in 'La Nuova' that the comune was looking to end discounts for tourists buying tickets in advance online, the argument being that tourists doing this already profit by not having to queue.