Unfortunately that wouldn't be a solution in my case. Theme choice can affect too much — from SEO to what features are on the site — for me to want to change them quickly. Besides, the larger customization is going to happen one way or the other. From a productivity standpoint it just makes sense to get it out of the way up front.
In the past it might have made sense for a brand new blogger to take that approach. But these day's I'm not so sure. First, free themes have become famous for their problems — from spammy sponsor links you can't remove to exploits that leave you open to hackers (with poor support and slow updates where the designer actually fixes these things). The old excuse used to be more about cost though. And these days that's not a good reason. If someone wants to start a simple journal-style blog, okay — go with a free blog host and theme and all that jazz. But if someone is doing it for their business, there's just no good excuse not to invest in something safe (whether it's a premium theme — which you can find for $20-40 at places like ThemeForest — or investing the time in learning how to customize things a bit up front).
Your design is a big part of your identity, and you have to be careful. When you blog as a part of your freelance business, you can't afford to have prospects think you look unprofessional.
In my own case, it also doesn't make sense because I have a fairly large reader base across my network of sites. And when I launch a new one, I usually promote it on existing sites and blogs. That means there's a surge of traffic right from the start, and the last thing I want to do is give them one experience and completely change it up on them a few weeks down the road. Because of that, I try to save re-designs for when they're needed due to changes in the site plan, when they're not conveying the image I hoped they would, when they break to the point where it isn't worth fixing them to work with new WP updates anymore, or when the blog is being re-launched after a significant break.
Just a few other things to think about when it comes to starting with a temporary design.