At Series A, you are talking about a lot more money and a lot more company history. So VCs have an incentive and the information to price and negotiate terms. And at that point, it's best to have a good startup lawyer on your side of the table because the terms will be many. A Safe is like training wheels for the complexities Series A will bring.
Really, no one has said it, but one of the few remaining competitive advantages of Silicon Valley is the legal talent available here. While I read Venture Hacks and Brad Feld as much as anyone, I'll get a good lawyer when/before I get to A.
Similarly, LLC's can be a simple stepping stone to Delaware C. Also, provisional patents are a stepping stone to a full application. Along with Safes, these allow people to move forward without the complexity and cost of completeness.
