>The vast majority of black holes have accretion discs, which have such high amounts of hard radiation, any probe would be fried long before it gets anywhere remotely near the black hole.

That's not true. Otherwise they should've been much easier to locate.

>And, using current spaceflight tech, it would take thousands of years to get a probe anywhere near one. So it would be an expensive investment, which might give us useful data in thousands of years.

*Hundreds of thousands

>Besides hard radiation, the only other really cool things we'd want to investigate are either gravitational (we don't have super-tiny gravity sensors to my non-expert knowledge) or quantum in nature (and I don't think we have a particle accelerator we can fit into a space probe.

Aside from the broken English,what¿?