[Hobadee]

You can make bombs out of pressure cookers. Since they are so good at holding pressure, you seal off the over-pressure release valves and then pressurize them until they burst (usually via some stupid or illegal means to begin with) then when they burst there is a ton of excess pressure. Big pressure = big boom.

[ReptileMan]

They are exactly as good as any other pot made of 18/10 steel with similar thickness. Other vessels like propane tanks are much better.

Boston marathon worked because - well pressure cookers at the time didn't draw much attention to them.

[walrus01]

But if you're not working with cutting torches and welding equipment, there's no easy way to open a propane tank enough to put an explosive in it (assuming purely amateur equipment) and then seal it up again good enough to hold high pressure. A pressure cooker on the other hand is ready made to have a big opening to put lots of stuff inside and then seal it up again.