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> something doesn't necessarily have to be true to have value.
I think this is correct. Many things have value even if they are not true.
However, I will give another point. Astrology can be called a form of divination (although, unlike cartomancy or dice, it is not one of the forms of divination that involves random selection). Divination can use artificial magical correspondences to represent things like you describe. Sometimes divination can make you to have some ideas about something, like you can do with cards (or computers) with words to make up an idea, etc. Of course, divination cannot be used to predict things scientifically or anything like that, and you should ensure you are aware of this when you attempt divination, so that you do not misuse it.
Specifically, astrology is a form of divination using astronomy (rather than cards or something else), and using some older conventions which are not used much in astronomy (such as naming the 30 degree arcs of ecliptic longitude after constellations which do not quite match them; this is kind of similar than how every 28-31 days in a year is given a month name; there are other conventions as well which are not used in modern astronomy because they are not useful in modern astronomy, but are commonly used in astrology). Astrology also involves astronomy and mathematics in other ways too; the house systems use geometry to map the horizon and midheaven at a specific location on the Earth, to the ecliptic; and modality/triplicities is like a residue number system (but with magic instead of numbers).
> I'm not going to suggest in any shape or form that A) all people born Cancers behave this way or B) planets have anything to do with causing this.
I mostly agree with you. However, anything can influence anything else, directly or indirectly. This does not mean that planets will cause you to behave this way, but if someone notices the movement of the planets and then writes about it, someone will read it, and writing about and reading about it is indirectly something that causes another; so you are indirectly influenced to write about this, and I am indirectly influenced to write this reply. Sometimes such influences may result in the behaviours described (like self-fulfilling prophecy), but many times it won't.
And, people might have multiple motivations and might sometimes do different things at different times, and of course you can do things independently of what was the ecliptic longitude of the Sun or planets at the time you were born.
Astrology is not bad, but it is often misused (although many things are misused). It is not scientific, but it does not have to be scientific in order to be meaningful. (However, science can also be misused, too.)