Well, they are still batteries after all, though a lot more lasting ones. But you can pry them apart and rebuild it, usually the batteries have their names printed on them. And they are still being made since they are used in modern powerbanks.

I don't really care to try and rebuild them; I don't need battery use out of these machines particularly. I'm just worried about them starting to leak if I leave them installed but dormant. The laptops sometimes sit in my closet for months at a time when I'm not using them.

I have a hard time remembering if I've ever seen a exploded laptop battery. I've had lots of DEAD ones, but I can't specifically remember any leaks. The thing here, most of these packs are sealed in plastic. So honestly the risk to the hardware is probably minimal.

I have seen a bad outcome for two Powerbook 540s which had main batteries left installed in them - massive corrosion which ate through the battery terminals and leaked across the whole case, battery cases were swollen with gunky residue at the terminals. Haven't seen that in any of my Thinkpads or Compaqs though, some of which are older than the 540s by a couple of years, but have seen backup battery corrosion in a Thinkpad which ate through speaker connectors.

I've seen things you wouldn't believe ... McBooks burst open courtesy of bulging batteries beyond the Tannhauser Gate ...

I cannot speak on NiMH, but I have seen old laptops with leaky NiCad battery packs that had corrosion beginning to spread to the battery contacts on the motherboard by the time I found them. If the only reason you want to leave the battery pack installed is for the weight or rubber feet on the bottom and don't care for actual battery runtime, then just crack the battery open, remove the cells and put in some dummy weight, be it pieces of metal or anything else heavy enough that'll fit. This will give you the same feel of having the battery installed but with zero chance of leakage.

If you really want to use to them without being tethered to the all then go ahead and rebuild the packs but it would be best to do it right the first time around so you don't have problems. As for modern macbooks they are not worth it at all and one is better off doing hackentosh with something like a Thinkpad.