1. You have no trouble hiring senior developers whenever you need one
2. You have no shortage of money to pay them
3. You are running a short term business, so your time horizon is only a year or so
4. You have no work that has been put aside as high risk given the uncertain benefits
5. You dev team is fragile, and liable to be gummed up
Then by all means do not hire junior people.
But if any of those do not apply, you will find all sorts of advantages. Look at the Google Summer of Code as a model, say. You get enthusiastic people willing to learn, who you can teach your way of doing things, and fast track their dev skills so they become senior sooner. So long as your codebase is slightly modular there must be something people can start on, and do not forget that teaching is a great way to help you think about things in more detail, and that meta thinking is the most important way in which you become a better programmer.