My instructor is very big on the spiritual aspect of Aikido, but he insists that the only way to reach it is through meditation. He believes that physical training is not enough. I think that meditation helps one relax and raises one's awareness, both things that are essential to master the techniques of Aikido. Wich is probably the reason why I struggle so much: I don't have much time to meditate, and my life is rather hectic these days.
O Sensei was a very spiritual man, and this can not not have influenced the style that he elaborated.
Of course, plenty of folks will storm this tread claiming to master Aikido without ever meditating. Maybe they don't realize they would be even better if they did.