Base saves
By a 'base save' I mean a save designed to be a starting point for progressing things in different ways - usually to produce finished (ready-to-play) saves. Each time you want a new progression, you'd load up the base save or a copy of it (from a memory card if you're using an Xbox), and do the extra work needed. If using a copy, you can then save. If using the base save itself, you can't save without overwriting the original, but you can at least play for a while before quitting harmlessly by ejecting the disc or switching the Xbox off. Either way, you'll have been spared having to do all the work embodied in the base save, so it's a potentially massive time-saver. You'll be able to produce a range of progressions with much less effort than producing each one from scratch.
For example, in the engineering of a battle where you have the opportunity of arranging Marines in different places, the sensible thing to do is to create a base save in which you've already done as much of the common work as possible. Each time you want to create a finished battle save, you'd start with the base save or a copy of it (if you want to be able to save) then do the necessary finishing work, putting the Marines where you want them and so on. Your base save frees you up to experiment and get a wider range of battle experiences; and you're consequently more likely to bump into the best battle experiences.
Base saves for base saves
In most situations a base save would be getting used as a starting point for creating finished saves. In some cases though, there's scope for having a base save that merely serves as a starting point for other base saves, which progress things toward different goals.
Here's an example. The spiral path megabattle and the twin bridges megabattle are able to share quite a bit of common initial work. You could have a base save that covers this work. You could then use this to produce (1) a base save that includes additional work relating to the spiral path megabattle, making a good base save for that battle, and (2) likewise a good base save for the twin bridges megabattle. Those might be the only two saves you ever produce from your first base save (you could then delete it), but you still saved yourself some time and effort.
If you get deep into the engineering business, you should be on the lookout for saving time this way.
Memory card capacity
Just so you know, a Halo save is pretty big! A 16MB memory card can store only four Halo saves, and that can be inconveniently few when you're in the engineering business experimenting with things. The convenience of a 32MB card would be worth the extra cost, I'd say.