I have both the TFI and MSD 6AL ignition along with a Weber carb, cam, and headers. I didn't notice a big difference in the idle but I could feel a small increase in power during acceleration. There was a small improvement in idle quality but nothing big. I am also using the MSD Blaster 2 coil instead of the TFI coil. For me it was worth it because I was given the MSD for free (it was broke at the time but due to a mix up at MSD it was fixed for free all I paid was $9 for shipping). It also helped out with the rich condition that I kept experiencing. If you can find a good deal on one go for it, if not and your idle is good and you are not sure about it then put the money somewhere else for now. I would say the price you pay and the present quality of idle and fuel burn will determine if it is worth it for you. Mine was running very well to begin with so that could be why I didn't see as much improvement. I also prefer to have the rev limiter on there just in case.
'85 CJ-7 - 258 bored .030 w/ mid level cam, 4.0 head, Clifford intake and header, Weber 38, TFI w/ MSD 6, Black Diamond 3" lift, Daystar 1/2" lift shackles, 33x12.5 Goodyear Duratracs with stock 3.31 gears.