TL/DR: This question presumes reading of a philosophical work for fun and self-learning.
How did philosophers err or underperform, and so enable or justify some skipping? Did they repeat themselves redundantly; digress; write poorly compared to other parts?
Optional Reading (The following only quotes advice on what to skip):
For example, Reddit's Flaired user 'ben_profane' advises on 2014/6/17:
'Heidegger and phenomenology are important and all, but a super deep understanding of them aren't really necessary'.
Reddit's Flaired user 'TychoCelchuuu' on 2015/12/5 advised:
I would not skip Locke's Second Treatise on Government. I am not sure reading Kant's Critique of Practical Reason is a good use of time. Thoreau could be cut.
Reddit's user 'paschep' on 2015/12/5 advised:
I think you should only read the three major parts of The Critique of Practical Reason, the rest doesn't really matter. Also I would highly recommend the critique of pure reason over all the other Kant works.
I think you should add Heidegger's Being and Time and the analytic philosophers (eg. Kripke, Quine).
I think you really should read the most important parts of these books, there are many parts that can be skipped.
Reddit's Flaired user 'irontide' on 2016/1/4 advised:
No, it isn't important to read a philosophy book back-to-back
[StackExchange User 'Timere' : The author meant 'front-to-back'].
There are very many books that are classics in the field but which almost nobody reads back-to-back as a rule but instead deals with extracts: two examples are Hobbes's Leviathan and Hume's Treatise. And reading them in excerpts is more charitable than not doing so, because these works are of uneven quality. People only care about the middle third of the Leviathan and especially don't buy Hobbes's claim that the physical theory in the first third entails the political theory in the second (and basically just ignore the last third, on the church), and Hume's Treatise is a big book of him flinging shit at a wall and seeing what sticks (not all of it does: the arguments concerning mathematics in there are pretty much an embarrassment, and his political philosophy in there is famously unconvincing). But the bits people do read is stuff so vital and important we teach it to undergrads, despite Hobbes's paper-thing arguments against his opponents and Hume's overweening love for his own opinions.