That's the point of it, they don't need to. The question is whether your _system_ uses -turbo or not, all apps will use the system lib. It's ABI compatible on purpose.
Well, I know everyone has said that it's supposed to be used automatically if you install it but I have had both of them installed for some time and it seems like not every app is using it based on the performance I've seen though the recent system updates seem to have changed that. I just - right now - fired up gThumb and some other apps after having installed xnViewMP - which uses libjpeg-turbo - as well as some system updates and now gThumb as well as some of the other image viewers I was using go through high-megapixel JPEGs from my DSLRs like a hot knife through butter, whereas it was a bit slow before I had installed xnViewMP and the system updates. Perhaps installing and using a program dependant on libjpeg-turbo finally made it work properly for others.
Sounds like something might be not quite right with your system. Multiple jpeg libraries can be only installed at the same time if they are using different ABI (for example jpeg6 ABI and jpeg8 ABI). Only one of them is actually used for real. The other one likely exists only to provide compatibility for the applications that are not recompiled with your primary jpeg library yet.
I'm using Ubuntu 12.04. After I installed turbo, I had tried to uninstall libjpeg through the Synaptics Package Manager and it told me that I couldn't do so without removing over a hundred installed programs and other things, which are tied to libjpeg's installation. v8 and v6 are installed for libjpeg. In going through the list of items it tagged for removal with libjpeg, it's pretty much the whole enchilada as far as system files go. The KDE, GNOME, and Unity desktops along with many regular programs - including almost all of the image viewers and editors - are tied to libjpeg being installed.
Last edited by TheLexMachine; 01-21-2013 at 03:39 PM.