Largely agree with your point on preventative actions being the same with or without testing, but there is a tail end that might warrant specialized action.
Having children is obviously a big responsibility, and every parent wants only the best for them. Doing genetic testing can at least feel like you've done your due diligence.
I would point out though that genetic diseases are a tiny tiny fraction of the things that can go wrong. The overwhelming majority of children turn out fine. But some don't. That's life I'm afraid. Genetic testing isn't a guarantee they'll be OK. I'd go so far as to suggest it's basically meaningless in that context (especially if you have access to parents, grandparents etc.)
However you progress, I wish you well.
So, in a sense, if you use a DNA service you are essentially putting your unique DNA marker "into the world", and it can be identified to you (especially if any of our relations have also been tested.)
While there may not be any immediate implications, that DNA might be used by law enforcement, insurance companies, employers etc (either now or in the future.) So there are potentially "high costs" - we just don't know what they are yet. But this bell cannot be unrung.
Balancing this possible harm are the possible benefits. Personally, I don't really see any compelling benefits, but my goal with this reply to the original question was to determine if I was missing something.