Ah, yes, the needle bearing 36X. I would love to be able to spout facts and stats as to why this, or that, friction coefficients between needle bearings vs. ball bearings... but...
Riley hit the nail on the head: More cost effective.
Some personal findings from my time at Fox as well as tinkering with Duke's big parts piles: I've never truly found differences between the two in the realm of performance. A good running 36X NB will run right along with a good running 36X BB.
There WERE some trade-offs in the realm of assembling the NB vs the BB. The NB required careful machining to insure concentricity between the bearing seat and the crank C/L. Also, in the rare event a NB should actually fail (seize), it was tough to extract it from the case and typically required a purpose-built tool for doing so. Upside: IF you had a failure with an NB, the bearings were captive and didn't damage the crankcase or crank. IF you had a catastrophic failure (the race breaks) of a BB, it typically trashed the engine because of the fractured race pieces flying about as well as the balls.
"Back when" I preferred the BB for psychological reasons, but now I had just as soon have a good running NB. Reason? See the above, and add to it: The NB cranks were ground .500" O.D., same as all the sleeve bearing 36 series engines. Because the BB had inside diameter of .500", the cranks for those were ground .498" to give a couple thousandth's clearance. Therefore, any .500" crank will fit my NB's, but very few (those ground slightly undersized) will fit in a BB, and typically it's a firm press fit.
SO... gimme' a good running 36X NB for the front of my vintage combat planes and I'm a happy camper.
* EDIT: With one exception. Forgot about the early-mid 70s 36 C/L Sport engine. Though ground to .500" O.D., the crank was shorter, thus won't fit a 36X NB.