The main reason is I found a beaker for about $20 less. But I'm wondering if decanting is more difficult with the straight sides.
Boiling would also seem to be less volcanic in that shape (I use fermcap but still baby those 2l boils)
Are you concerned about keping the starter sanitary during the propagation of the yeast with such a wide mouthed vessel. That's a good price on the beaker though. Is it borosilicate?