That is absolutely not what I meant. I've been using messenger bags for years and what you are describing here is not because of their messenger-style but because of them being bags. In other words: every bag will have the problem you are describing, stuff will move around which is why we have things like dividers and pockets. Thinking about what goes where does help too.
What I meant is that it seems that the Timbuk2 bags need to be more carefully packed compared to other messenger bags to keep things like rain and snow out of the bag. Not a very big issue though.
No need, already found one weeks ago which works beautifully against rain (have been able to test it already

) When I was still looking for a bag I noticed the strange (well at least to me it is strange) design with the Timbuk2 bags and I wondered if this was going to be a problem or not. It seems that it is not a big problem but you need to be a bit more thoughtful.