Or you could look at it as them believing Naruto to be strong enough to handle the 9-tails. Remember all the failed hosts in the cloud and sand villages? This isn't a trouble-free power up they're handing their son, it's an extremely powerful monster that could break out, killing the host.
However, I do get what you're saying; Minato essentially believed that without the 9-tails Naruto wouldn't be able to defeat Tobi. To which I reply, from what we've seen Minato would be an idiot if he thought otherwise. There are literally no other ninja in the entire 80000-strong shinobi army who could:
a) Detect the zetsus.
b) Take out the edo tensei across multiple battlefields at once.
c) Fight tobi evenly, with Bee's help.
At some point, belief in ability becomes arrogance and stupidity. Believing someone without the nine-tails could fight Tobi is waaay past that point.