My personal reaction to starting with a dream sequence is that the author, either consciously or subconsciously, thinks the real-world opening is terribly dull. So s/he punches it up with a gripping opening scene, free from the constraints of "reality" (fictional reality, at least), in the hope that readers will hang in there past the moment they realise they've been hooked with fake bait.
At this point the reader usually discovers that the real-world opening which follows is terribly dull. Usually somebody wakes up. Then there are generic breakfast and going to school scenes. If we're lucky, something interesting might arrive by the end of the chapter. But probably not.
My (strong and bitter, like coffee) opinion is that authors who can provide a gripping opening they can actually carry through, instead of faking out with the "it was all a dream!" ending, do provide such an opening. Everyone else does dream sequences.
(Death! Fire! Destruction! Dream sequences must perish!)