I've been bowling for almost 5 years now. The first three years I bought 3 balls. Since December 2014 I probably got 20-25 balls between NIB and first drill.

When is a ball I absolutely need I get NIB, otherwise if I just want to try it I go with first drill.

I don't! I am a ball hoarder lol 

How many balls do you buy a year?
Last year I probably bought around 10 NIB.  The majority were for my son who went from 14 to 15#. He is a tournament bowler so carries a complete arsenal (and he has a Dad who can't help himself).  One new ball so far this year (Carnage...for the boy).

I also buy some used and/or trade used for used.  My son carries two of those.  One is an orange recon; a ball he really liked at 13#.  I was fortunate to find a SD in very good condition.  The other was a Tag that was a very good price and already plugged, so it was worth a shot.

I am more apt to go a used route for myself, but prefer it to be already plugged.  When my son got the Carnage, I took his Guru Master for myself (his only non-Motiv) since I don't have a heavy oil ball.  It is currently being plugged and drilled.

Why do you buy NIB'?
I generally buy NIB for the boy with a couple exceptions.  Usually because it's a newer release.  Also, no matter what I need to plug used balls and have them redrilled. This tends to offset the cost savings enough that just paying a few more for new makes sense.

Do you actually need it?
Do I need it?  Probably not, but I get NIB balls and have them on the rack in the basement for future use.  These would be NIB balls that I find great deals on that also would qualify as future replacements.  My son needed them to fill a full arsenal for a variety of house and challenge conditions.  I don't sweat getting him what he needs to succeed.  With the Carnage he is now set for a good while.  I don't anticipate buying as many balls this year.  Only when things need replacement at this point.

First I bought was 1 year and 5 months ago.
Then 2 months maybe went by and I bought a spare ball.
Then it a few months went by and I bought the Jackal. That was 3 months ago or more.
Now I'll be buying 2 more balls. Swerve and Scandal(well if the PSO haven't drilled the balls I ordered from him first. I don't want the Menace anymore, would prefer the Scandal).

I probably don't need any of them, but I want them.
Though I read on here that the Scandal is for rev challenges players, which I definitly am
And I want to try out the hyperSHOCK tech in the Swerve. Though these days most balls probably go through the pins just like the Swerve.

the last 4 years or so I was a 1 ball a year guy.

This year I have bought 3; due to a switch from 16 lbs to 15 lbs.  and with my birthday coming up and a nice potential deal on a NIB fight it just might become 4 NIB balls this year.

I rarely buy used; usually a used ball is an experiment of some sort.  like a different drill pattern, or an attempt to get me to expand my horizons on brands.

as for "do I need it?"  on tournament days I think I do.