from my experience I can tell you that these four (yahoo, dmoz, technorati, digg) is worth the effort.
To me DIGG and all other submission-type social networks are just the same as webdirectories. The whole submission process is,however, much faster.
Where in the world are you getting your information, basically what your saying is that if I submit a competitor web site in some "dangerous directories" I could have his/her site penalize? I don't think it works like that at all, other wise there would be a lot of pist-off webmaster going on around, that would be very good for web directory owners, just imagine the submissions free and paid just to help bring a competitor in a sandbox. You have no idea on what you're talking about, you've read this from the so called "seo experts" and for some reasons you now think you are one. lol As I mention a couple of times that you and a few like you simply do not understand what web directories is about, because if you do, you would not be bashing it, it is not the software or the scripts that make the directory to be "useful" and or "dangerous", it is in the hands of who ever is administering it, if it becomes a useless tool, then it is useless, but you cannot go around saying it to the whole that it does not matter much, because there are thousands of seo companies that use them and the company they submit are: Virgin, British Airways, Walmart, Shopping.com and the list goes on. Run your own web directory and get it to a point where it attracts a well established seo companies which have big clients and you will know that web directories are one of the tools they use for seo. If you have not run a directory and know nothing much about it other than wherever had you read it from, then keeping the lid shut is the smartest thing you've done.
directories are good to get a new site indexed but for initial rankings there are better options but they should always be part of your overal plan. if you find it time consuming, do something which has a higher impact