"It's okay to occasionally split an infinitive in proper English. Sometimes, it prove helpful when one is trying to clearly state one's case."
I don't think I have ever fully understood or even necessarily agreed with the "never split an infinitive" rule, and I certainly don't understand the difference as to when it might or might not be acceptable to do so. Seems to me this is one of the grammatical rules that might be considered more of an opinion, like whether or not it's okay to end a sentence with a preposition. Still, I recognize that many people do consider it a hard and fast rule.