Wouldn't a simple rounded tip cylinder be good enough? Why does it have a diamond shaped arrow tip?
The tip has to push switch noses and contacts out of the way when being plugged in and out. A linear slope translates into linear movement, predictable sideways forces and limited forward forces. An unspecific "rounding" will tend to "burst" through side contacts and switches, leading to more strain on them and making the plugging action less well controlled.
I've seen both rounded and cone-section tips on 1/4" jacks. Both are compatible with the usual designs of the receptacle (which have a spring clip inside that fits the recess between the barrel and the tip of the connector).
A cone section is cheaper because it's easier to machine than a sphere. The same will apply for 3.5mm and other jack variants.
You also need the recess between tip and ring. The spring clip fits into this recess and provides some degree of anchoring so the connector doesn't fall out (important e.g. on guitars, where the receptacle often points downwards).