[Disclosure: I work for AgileBits, the makers of 1Password.]
To give credit where credit is due (comics rarely have citation footnotes), this scheme is called "Diceware" and should be credited to Arnold Reinhold, who described it in 1995.
What is absolutely crucial to the scheme is that words must be chosen from the list by a uniform random process. (Reinhold's scheme involved rolling dice.) Humans, particularly when trying to be clever in selecting passwords, are very much not random.
Capitalization and symbols (the actual question)
hashcat, as it stands today, isn't good at applying transformations (e.g., case changes) to guesses that are done by combining words (needed for Diceware=like systems). So in once sense, using a few symbols and mixed case for these sorts of password will really make things hard for hashcat. However, that would be a stupid reason to do something like that. After all, a week ago, just using a passphrase longer than 15 characters would stymy hashcat.
It is unwise to base a password selection policy on potentially transient idiosyncrasies of a particular cracking tool. It is important to realize that other password cracking tools, John the Ripper, for example did not have that 15 character limitation. And has we know, these idiosyncratic limitations do change.
Look only had how many guesses are needed
The Diceware list contains 7776 words (you get these through five rolls of a die or one roll of five dice), and so a password would be about 12.9 bits per word. So a five word Diceware password would be about 64 bits. If hashcat could check 300,000 per second then it would be about 1.5 million years to crack a five word diceware password. How many guesses per second hashcat can achieve (on the same hardware) depends enormously on how the password is hashed (straight hash, PBKDF2, scrypt ...).
The reason I'm picking 300,000 guesses per second in my example is because that is the number we get against a common form of the 1Password Master Password (PBKDF2-HMAC-SHA1 10000 rounds), which I wrote about back in April. There are more numbers in that article, along with links to other articles on using Diceware (including the article, Toward Better Master Passwords that may have triggered revived interest in Diceware just prior to the XKCD comic, and advocates a modified Diceware system for the few strong passwords you need to remember.)
hashcat character limit
Hashcat's move from a 15 character limit to a 55 character limit only matters if you were specifically choosing a password greater than 15 characters to avoid being in the domain of something that hashcat could work on. I hope that nobody was doing that.
In that article I wrote back in April I studiously did not mention the 15 character limit (which I was fully aware of) for the simple reason that I wanted to offer pessimistic (from the defender's point of view) estimates of cracking time weren't relying on what I saw as temporary limitations.