The Lord of the Rings films had the same things going on; several of the cast members mentioned looking outside their hotel room in the morning to find a giant stack of rewrites from the previous night.I'm going to use the word "ignorant" here nut not in its pejorative sense.
From many of the posts I've read, here and elsewhere, it's obvious to me that the people who speak about the script delays and production start dates are ignorant of the reality of how movies (and TV for that matter) are made.
It's not impossible for a film to have a "draft" script in May and start shooting in July, for instance. Pegg could have turned in a messy draft script that had most of the story beats worked out to the satisfaction of the producers and studio executives, which they could then approve for a rewrite/revision with very specific notes. For instance, they dialog might not be final, and all the action in specific scenes not worked out (I don't mean big action set pieces necessarily), but if there story structure and beats and where the scenes take place is solid and not likely to change the studio can and will start preproduction on those elements, from set to costume to prop design. Scripts are rarely set in concrete during pre-production, and scenes get tweaked, massaged, added and even omitted right down to the wire. That's the nature of the business.
So, yes, more prep time is better, but just because they don't have a locked production draft today doesn't mean they can't possible start shooting two months from now.
The original Battlestar Galactica series was plagued with lots of such production problems. A lot of times, there were even mid-shooting script changes. That's why some of the eps seem to lack a bit of cohesion.
But, for what it achieved, the original Galactica still became a bit of a success story even if it only got the one season.
And we're all familiar with TMP's production rewrites being such a nightmare that cast and crew were getting new pages not just daily, but sometimes HOURLY, with annotated timestamps from either GR or HL. I don't think I've ever heard of a production that's filmed from day one with a locked script without a single rewrite.
If the money and the talent are there, things will get done.