Rik said:50 off 50 balls is still 50 off 50 balls never mind if you score them all the way through your innings or you go mental at the end. Once he gets his eye in he goes for it, but he still ends up with a good strike rate despite starting slow as you say he does.
From my opinion momentum is a moment sort of thing. If he slows it up he slows up the team therefore increasing chances of a lower score, no matter how beserk they go at the end.