What is the benefit of that extra effort? Flow.
When you do a ritual, ideally you get into ritual consciousness. This consciousness occurs when you're brain is in the Alpha state. Just like when you have a standard procedure that you get used to, as I discussed in the previous blog entry here, to memorize the parts keeps things running on auto pilot. It's spiritual cruise control.
When you don't have anything memorized, and you have to keep stopping to refer to your book or cue cards or whatever, it can really break a person's flow. It brings you in and out of ritual consciousness so you never get deep into it.
When your circle casting is memorized, much like the way you memorize your favorite song, it just flows out of you, as does your ritual.
My Tip for You Today: Try memorizing parts of your ritual.
Start with one thing, such as your circle casting. Memorize it before your ritual, then review it before rituals for a while until you're sure you have it down. Sing it if it helps you remember it. Spend the next month or two casting your circles without reading it.
You'll probably notice how much more smoothly things go. You'll be amazed at how much flipping through pages can interfere with the spiritual mood.
I'll bet you find it's worth the effort.
Do you have any parts of your rituals memorized?