Titanium is indeed fairly soft, but softness doesn't much matter for aircraft since they don't tend to get scratched or abraded. Tensile strength is much more important.
The blackbird was not made of titanium because of how hard or how strong it was, though, but because titanium maintains its strength at the high temperatures which happen to aircraft flying at mach 3. Steels start to become weak at very high temperatures. Ti-6-4 (the most common titanium alloy which I believe is what the blackbird was made from) is almost 3x as strong as stainless steel at 1,000 degrees F, for example.
I actually have a titanium wedding ring, almost 20 years old now, and it's scuffed and scratched a bit. I wouldn't say it's any worse off than gold which is a softer metal, but since it was anodized you can tell the scratches easily. Ti is a bigger hassle to work with not really because of its softness but because it tends to gall when machined, and because it generally has to be welded in a vacuum. It's basically a big pain in the ass.
I'm a mechanical engineer who used to work at a titanium smelter, FYI. I hope I don't come across as a know-it-all, I just love learning and talking about metallurgy. (But I would still never buy a titanium cutting board!)
