Hi Phil,
a very sensible question but unfortunately only you can answer this one! I could not get by without mine now but everyones situation is different
Wether you need a lathe or not depends on a few things, firstly, how deep are you wanting to go down the rabbit hole 
If like you say, you want to make your own balance staffs you need to have a lathe, unless you fancy learning to use a turns and a bow.
Re-pivoting? You will need a lathe.
Pivot polishing? Get the jacot drum with the lathe and you can do this too otherwise you could just do it with a jacot tool.
I use my watchmakers lathe almost on a daily basis and I dont even do watches yet. I have been learning to turn balance staffs. One thing I would say is even if you did buy one, its not like you will lose any money on it as they just keep going up and up in value.
If you do start cutting balance staffs the lathe will pay for itself very quickly, even if you dont, you will still find it useful for so many things, just recently I was restoring an old clock which had a brass knob on the door that was missing. Turned one up in about 5 mins.
Good luck whatever you decide
Paul.
