The biggest problem with running multiple accounts is a UI issue. Many services (such as Facebook) assume you will have only identity and make it difficult to switch between accounts because they save some state (setting a cookie). But one can use separate browsers or install a browser extension that makes it easy to switch sessions. And many third-party apps (such as Hootsuite) recognize users are likely to have multiple accounts. Heck, the latest version of the Twitter native client allows multiple accounts and even Google is making it easier to set up and switch among multiple accounts.
We just need more developers recognizing that we have multifaceted personalities and accommodating multiple personas for our presence online. No harm in that.