Now let's also say for arguments sake that when you interrupt your MG after a tick - wherever it is - you lose around 0.3 seconds due to natural human reaction time/latency.
You need to refresh Corruption. You can refresh it any time during its final 8 seconds (random number picked for this example) at level 90 and suffer no penalty.
You are channeling Malefic Grasp and notice that Corr can now be refreshed, and UA can be refeshed in 4 seconds. You channel it for two seconds, and interrupt to recast Corr. You then channel for another two seconds, and interrupt to recast Unstable Affliction.
In total, you've cast 4 ticks of MG and you've refreshed Corr and Unstable Affliction. You've done 4k damage total and refreshed your Dots, and suffered a total of 0.6 seconds wasted to MG interrupts.
You are channeling Malefic Grasp and notice that Corr can now be refreshed, and UA can be refeshed in 4 seconds. You channel it for its entire cast and then refresh Corruption and UA together. You have channeled 4 ticks of MG for 4000 damage, refreshed both of your DoTs, but have wasted only 0.3 seconds.
Also, this exact behaviour is modelled in Simcraft. So no, my common sense does not fail.