It's a stepping stone of sorts that works out all sorts of kinks and is WAY easier than actually dumping the crew into an alien gravity well and then extracting them back. Plus it gives all sorts of other useful data, like if people go nuts being in zero G for that length of time, or if we even have the capacity to make a trip of that length, the effects of solar radiation outside the earths magnetosphere, and a bunch of other stuff.

would be enormously expensive.

Un-manned rovers and close-by satellite telescopic cameras can give us more information much cheaper.

As a simple practical question.

How much would be the total initial weight of all the food and water required for 2 or more persons for 501 days?

Those incremental steps made sense for going to the Moon, but there's a huge difference between going to the Moon and going to Mars. You can go to the Moon and back in a matter of days, and you don't have to wait for a window that only occurs every 18 years. If we do a flyby mission to Mars starting in 2018, it'd be nearly 2 decades before we can do a similar mission and actually land.

We have lots of zero G experience from MIR and the International Space Station, where people have stayed in zero G up to 437.7 days.

https://en.wikipedia.org/wiki/List_of_spaceflight_records#Ten_longest_human_space_flights

If solar radiation is going to be a problem, it'll be one whether they actually land or not. In any case, if you want to measure the effects of solar radiation outside the magnetosphere, you can do that without going all of the way to Mars. You can do it nearer to Earth so that it's possible to retrieve them if they do start to get sick.

Expending all of those resources and time testing things you can test in more efficient ways, and wasting a once in 20 years window, doesn't make sense to me.

Mars launch windows occur every 2 years. We would not have to wait till 2031 to land on mars. We would only have to wait till 2031 to do this exact same type of flyby mission in 501 days. Tito is utilizing a rare earth/mars alignment to accomplish the flyby in the shortest time possible. There are rockets being tested today (VASIMR) that could conceivably get us to Mars in as little as 39 days.

I don't think it makes sense to expend all of those resources and go all of that distance just to do a flyby. We've robotically gotten to Mars and landed successfully a number of times. If there's concern about whether a craft can land and later successfully return to orbit, we can test that robotically. We've tested the effects on people of hundreds of days of zero g. We can test the effects of cosmic radiation without going all the way to Mars.

We should get all of the testing and experimenting done here, or nearby, or on Mars robotically. Then, if we send people to Mars, we can land.

But I think this mission has value. Do not underestimate the effect of having humans go farther into space than any human before, and actually lay eyes on another planet even if they don't actually land. It will stir public imagination and may just give a push to efforts to have an actual mars mission. I would prefer a landing too, but for a privately funded mission (funded by a single man no less) this is about as good as it gets.

than anything expected from current technology.

This is more like the (unplanned) Apollo 13 flight - get there and come straight back. It would give some data on surviving radiation, but having to do that in such a hurry, to take advantage of how close Mars will be for that orbit, may make that of limited use. If they want data, it'd be better to set up a system with a few test animals in varying shielding conditions, so you can compare them. This way, it'll tell you if whatever solution they select works. Or not. The 501 days isn't that much longer than the record 437 days Valeri Polyakov spent in Mir (in one go; he'd already done 240 days a few years earlier).

I can see some data will come from it, but it seems mainly a "we made it to Mars and back alive" adventure.

Start with Mad Tony Scalia.