Logic probe circuit; what is the use of some of his components?
The point of both diodes and capacitors is stretch the output to make LEDs detect even very short pulses. Even if there is a very short "high" pulse, then C2 is quickly discharged through D3, and once the pulse disappears, Cw charges through L1 causing it to glow. The same happens with "low" pulses and C1/D4.

Since you have removed these components, theoretically, you will no longer be able to detect very short pulses (less than 10 microseconds).
I do, however, question the design -- 0.1uF driving 10mA LED gives time constant on the order of tens of microseconds, and since 74LS series do not drive very strongly, a sub-microsecond pulse will not discharge the capacitor much. So I am not sure you are missing much.