It's sometimes difficult to scan vertically the beginning and ending of each post. This is because there isn't much of a visual separator between posts. This becomes even more challenging when you have several quotes in successive messages.
While this isn't such an issue with reading through the messages, it is a challenge when you're scanning a thread to find something you've seen before and want to find again. It also comes into play when you see a conversation between individuals you want to skip and quickly find the place where the conversation is relevant to what you're seeking.