While it is certainly not neccessary to have a master's degree (I entered directly into my PhD program) there is something to be said about spending the extra time. For instance when applying for funding, a 1st PhD with a 2 year masters may have a sinificantly better application (ie publication record) than one without. The same will apply to you when you move on to a post-doc. So considering how condensed your program will become, you should weigh the pros (finishing the PhD faster) verses then cons (not accomplishing what you need to to get the great post-doc posistion). Whatever you choose tho, good luck!
