The guillotine has its name due to the fact that it is used in the guillotine submission.
The weakness of this grip is that it uses the strength of one hand only to grab on the forearm, the other hand’s grip is completely nullified.
This grip does still have significant strength but it doesn’t use both hands’ strength in full potential, therefore it is not recommended to use it instead of a gable grip or an S-Grip.
Sometimes in a fast and explosive grappling exchange, you don’t have time to establish a full gable grip or S-Grip, you just grab whatever available to connect your hands together.
Also in many grappling situations, it is not required to establish a grip with Hulk strength, some grappling techniques require the minimum strength and rely more on the technicality of the move.