1) your bank account is not a person or a point of view (I will accept debate on the latter part of that statement)
2) worthless to you, but is is worthless to the bank?

:D

If all else fails you could use people for meat or biofuel, right?

1) No, but you are the one who introduced the concept of negative and positive value. People have intrinsic worth as human beings (at least MOST people do), but their opinions may have little or no value. (say, haven't we been here before?). I am not interested in the opinion of a blatent racist who screams for the deaths of all members of some ethnic group, for example. Nor am I interested in reading trivial statements made by people who are just looking for attention, and make their statements out of ignorance and/or lack of ability to use logic and intelligence. Why waste my time?

2) The bank is out $300 of its money. If I don't replenish the account, it has value to the bank as a tax write-off, I suppose, but that's it.

The account may be worthless to you when it comes to buying food, but it still exists, it still involves dollars, and you still have to pay it off.

We're still arguing about what mijsnomis meant when he used the word "validity".

There's a few possible meanings

1. a person has a point of view
2. that point of view has some basis in reality
3. every person's point of view has value to someone
4. it is beneficial to a person to have a point of view
5. it's worth my time to hear someone else's point of view

The first is almost always true. The second, according to wolf earlier in this thread, is almost always true. I assume the fourth is true. The third is implied by the fourth.

The fifth is up for debate.

I think that, according to the purely subjective view, it is always valuable to learn someone else's point of view.

According to the objective view espoused by smoothmoniker and wolf, it's not always worth your time to learn someone else's point of view. The other person could be a complete dumbass, and learning their point of view would not only be difficult, it would have negligible value. They could be a psychopath, in which case absorbing their outlook on life could be damaging to you -- negative value.

Or, they could be an entrepreneur or a scientist, and learning their point of view is valuable. They could be a teacher or a tutor, and their knowledge is not only valuable, but they strive to make it easy for you to learn it.

Deciding which people are most worth learning from depends upon having some outside measure of value.

(Edit: added / to close the link)