Hi Mike...
I am sure that you are always going to get a range of opinions to this kind of question.
I think that for your site the answer may be simply that having a robots.txt file is more of a “belt and braces” safe harbour-type thing – the same goes for say whether you should have a keywords meta tag – many say these pieces of code can be of marginal value but, when you are competing head to head for a #1 listing (ie 35%+ of the clicks) then you should use every option and weapon possible ...furthermore, if your site is likely to grow significantly or eventually have content/files that you may want excluded, it’s just a “tidy” thing to have had indexed over time.
Also, don’t forget that best practice robots.txt file taxonomy is to also include directions to your xml sitemap/s.
Here is an example from one of our sites...
User-agent: *
Disallow: /design_examples.xml
Disallow: /case_studies.xml
User-agent: Googlebot-Image
Disallow: /
Sitemap: http://www.sitetopleveldomain.com/sitemap.xml
In this example there are two root files specifically excluded from all bots and this site has also specifically excluded the Google Images bot as they were getting a lot of traffic from image searches and then subsequently seeing the same copyright images turn up on a hundred junk sites – this doesn’t stop image scraping but certainly reduces the ease of finding them.
In relation to the “or 1-line file giving all bots all access” part of your question...
Some bots (most notably Google) now support an additional field called "Allow:"
As the name suggests, "Allow:" lets you specifically indicate what files/folders CAN be crawled, excluding all others. However, this field is currently not part of the "robots.txt" protocol and so not universally supported, so my suggestion would be to test it for your site for a week, as it might confuse some less intelligent crawlers.
So, in summary, my recommendation is to keep a simple robots.txt file, test if the Allow: field works for you and also ensure you have that guide to your xml sitemap – although wearing a belt and braces might not be a good look, at least your pants are unlikely to fall down