It has poor test/retest reliability because the constructs it tests for can be mood dependent rather than traits. It has poor construct validity because the things it measures aren't necessarily measures of what it says it measures.
I'm not going to disagree with you there.
It's bollox.
By the definition above - potentially. On a social level, I think any frame of reference based on individual input (rather than astrology etc) that helps people understand both themselves and others is useful. In fact, more so to put it in common terms rather than scientific ones.
And it's frequently used as a stick to beat people with.
That I obviously think is a bad idea. But we did DISC testing at work, and it's largely been positive in understanding what makes each other click.
The man who had the biggest blindspots regarding his own weaknesses was also keen to wave around the 'it's bollox' flag, when his results described him to a tee. Right down to the fact that he thought my profile was more appropriate to him, when everyone else was laughing at how well it defined my approach and attitudes. (right down to the fact that I was complaining that it claimed I had great attention to detail when I thought can be sloppy, and they pointed out my 7 page minute by minute day plans for events colour coded by room, and that it also said 'Cats can hold herself to unrealistically high standards').