How important is a preamp for purely digital sources?

I would put the money into getting a good DAC, since the DAC in the Node2i is nothing special.  Many DACs can be digital pre amps.  My personal experience 
was when my Mytek Manhattan DAC was doubling as a pre amp when the latter was having a minor repair issue.  I definitely preferred using it with a pre amp as I found the System had some glare without it, but ymmv.  Of course, many pre amp DAC combos are out there as well.  I would spend the money on a DAC now if the budget is small, and you can add a pre later if desired 

I was using ADI RME2 DAC as preamp (you always have some kind of preamp, passive or active, i suppose. If you're relying purely on digital volume control of DAC, that's the worst scenario since it reduces S/N ratio greatly. Try to avoid that at all cost!). 
Later I switched to tube preamp (Primaluna), and difference was significant. Another upgrade was replacement of ADI DAC, and that brought much more musicality, so go for R2R or mutlibit DACs rather than delta sigma ones (general advice, there are great delta sigma dacs of course!)

The knock against digital volume controls is no longer true.  If properly implemented they will not strip bits or increase noise levels when utilized in appropriate setups.

I agree with Hudo. I have a modwright swl 9.0 tube preamp and the improvement was stellar. Adding the Pontus dac a couple a years ago has made it hard to leave the house sometimes. If you do add a preamp, make it tubes. I have a solid state amp, and the combination of the two really sings.