We have to question what "understanding" it is that we're evaluating: are we looking only at the understanding that Montresor has of the situation, or some kind of objective truth?
Montresor is a classic example of an unreliable narrator, specifically because he never tells us the nature of the offenses that Fortunato has supposedly committed. We do not need to know their nature in order to understand Montresor's anger and his need for revenge; however, we would need to know them in order to understand a more objective perspective. For example, is Montresor's revenge a suitable one for the offenses that have been committed? Can we consider ourselves to be more like Montresor or like Fortunato, on average, given the specific circumstances? Is Montresor a noble avenger or a murdering psychopath? These issues might be answered, but since the evidence that would contribute to them is left unspoken, we are forced to focus only upon Montresor's emotional perspective.
At the least, we are aided in understanding Montresor because we do not muddy his emotional perspective with Fortunato's - we don't know if Fortunato would agree with, or even be aware of, the charges that Montresor has leveled against him. We do get a glimpse of this at the end, when Fortunato's silence makes Montresor "sick at heart" - perhaps indicating a guilty conscience or an excess of empathy that Montresor needs to steel himself against in order to go through with the deed.
So, I think the focus on Montresor's perspective is meant to help us understand only that perspective, rather than an objective truth.