When I first heard the word "diarise" many years ago I had that uncomfortable, priggish feeling that one gets when lazy abusers of language make up ugly constructs. But over time I have slipped right into using it because I can't remember what we used to say before! "I'll make an entry of that in my diary"? Please, who has the time?

For the same reason people use "action" as a verb; it takes half the time to say. I haven't yet succumbed to that usage but past experience shows that I will eventually lose my sniffy attitude and fall in with the mainstream. I just don't have much staying power as a zealot. (sigh)