The cross-pin slide has the shift clutch cross-pin (#62) passing thru it. The smaller-diameter portion of the slide contains the shift clutch spring and on the other end, the shift cam follower (#54) pushes against the slide (and consequently the shift clutch) to shift to Neutral and Reverse.
It definitely is an essential part. Without it, the clutch spring sits against the cross-pin, and it's not designed to do that. And the shift cam follower is pushing against the cross-pin, which poses (2) problems: first, the cross-pin has a lot smaller contact area than the end of the slide and likely will wear out if the slide is not installed; second, without the length of the slide in there, the shift clutch won't travel as far as it's desgned, when you shift to Neutral and Reverse.
You may feel that the gearcase is shifting OK, but if the shift clutch isn't pushed all the way rearwards (as it's designed to do), the clutch dogs may not fully engage reverse gear, whch can cause the teeth to jump & damage the gear/clutch.
I wouldn't recommend running that lower unit without all the parts that it came with from the factory. It's just inviting trouble. The 30624 cross-pin slide is NLA, but fortunately there are a number of used & new parts listed on eBay at this time:
http://www.ebay.com/sch/Boat-Parts-/26443/i.html?_from=R40&_sop=15&_vxp=mtr&_nkw=30624+slide
The $12 shipped slide & spring look fine to me, I wouldn't have any qualms installing those parts. Be sure to check the shift cam & follower for wear and replace as necessary. Otherwise you could have similar shifting issues as with a missing slide.
Hope that helps........ed
