You don't need external temperature sensors to gauge temps in processors, harddrives and motherboards. Each hardware already has them. The software reads temperature from the temperature sensor embedded within the hardware and not from the case's temperature sensor.
^^ Id on't think that these sensors are that much accurate - internal digital thermal sensors embedded in cpu/mobo/hdd are much more accurate - granted external sensors are good but accuracy depends on the placement of these external sensors - for eg. on the screenshot you posted HDD temp i similar but what about cpu temp ?? it's 8c higher in software reading.
I use two HDDs, I haven't attached the even one sensor to CPU.
My HDD sensor is attached to the surface of First HDD(SATA) and CPU sensor is attached to second HDD(PATA).
use some temp monitoring app like HWinno/HWMoniotor to get a lot more sensor infos including volts/rpm etc and they have log feature and some of them can show temps on the screen while gaming and some logitech gaming KB supports app like this - you can get the sensor infos on KB display
Means you don't have to open up a program while playing games to check the temp, or while doing some hard end stress test or while encoding on an overclocked processor.
I don't think this is not worth it, but whatever all that matters is individual choice and preference. Thanks for discussion.