There is a very lightweight spring assembly on the rocker arms that slips into the pushrod tube hole. It looks like it serves no purpose other that to provide a little pressure to hold the pushrod tubes in place. Is this necessary?

absolutely necessary

it holds the pushrod tubes into place as you said
without it they will vibrate loose and you will have oil leaks

absolutely necessary

ABSOLUTELY!!!!!!!!!!!

Leave em out and be prepared to lose your oil.

They hold the tubes in place. without them the tubes will, and I emphasize the "will", back out.

Advice taken, It seems like they are pretty wimpy springs to hold the tubes in though.

When installed correctly they "Snap" firmly into place. If yours don't feel secure then they probably aren't in right.

some pics of them installed:

http://www.914world.com/bbs2/index.php?showtopic=207443

Stu

agreed
its not that tricky...
but its an extremely common mistake to not get them correctly in place
and then the oil leaks begin on your brand new motor.

make sure you have them snapped correctly into place

I don't have them on mine. Haven't for 25,000 miles. No leaks.

Six-cylinder, Elliot?

BTW, it is not uncommon to leave the bent-down parts of the retainer sticking down inside the pushrod tubes. They will wear a groove in the pushrod if you do that, in addition to not holding the tube in place.

The bent-down part of the wire needs to sit on the shoulder of the tube, so it presses it inward.

--DD