My question refers to the 8th Level MU spell Binding found in Unearthed Arcana. In it, it describes the ability of other mages to enhance the power, essentially, of the primary spell caster (up to six assistant mages). Can anyone explain to me, in 1st Edition rules, how this could not be applied to all MU spells (or Cleric spells too, for that matter).
Well, it's 1st edition rules. The simplest explanation is "because it isn't in the rules".
AD&D's magic system isn't very clearly defined, with the abstraction of spell slots being hard to apply to in-game magical metaphysics. That's why we get questions like this, and that's why we got the very awkward mechanics of metamagic feats in later editions, just to make this unwieldy mess a bit more flexible.
Specific settings would sometimes make magic a bit more coherent with regard to in-setting metaphysics (Dark Sun, for instance), but even then, a lot is just accepted as is.
In my experience, lacking a specific spell for a job either means the system doesn't want you to do it, or that you can research/find a spell to do the job.
Ask your DM if he is willing to allow you to either find a spellbook with the spell you are looking for in it, or maybe buy it off an NPC Magic Guild (if they exist in your campaign), or whether you can research it.
A lot depends on your DM, and whether they think that this spell will either be too powerful or simply not a part of how magic works in your campaign world.
The other thing to consider is spells that have a similar effect in other editions. AD&D 2e is probably the most similar, and it contains a priest spell called 'Combine'. Your DM may allow you to convert it to first edition, and maybe create an equivalent wizard spell. This is quite likely, as a lot of fantasy literature involves ceremonies whereby a single person can use several people's magical power.
From reading the spells it looks to be evocative of various fantasy novels of the day and past that had cabals of sorcerers summoning up a demon. For example the medieval depiction of witches sabbaths. In this case it is written as a specific feature for this spell. Not as a general rule.
As a house rule I used the following for my Majestic Wilderlands campaign. The assumption of my setting is that standard D&D magic is the currently the most advanced form of magic. There are other forms of magic but have disadvantages of one kind or another.
In addition I have a rule that magic-users can cast as a ritual spells from their spell book. The highest level they can cast is 1/2 of the highest level they can memorize rounded down. A magic-user capable of casting 5th level spells can cast upto 2nd level spells as rituals.
The ritual takes 10 minutes to cast, and requires component equal in cost to the spell level squared times 10. A first level spell cost 10 gp to cast, a second 40 gp and so on. The only limit is the fact that it takes a turn (10 minutes) to cast and amount of gp you are willing to spend on components. The turn casting time means that players only use ritual for support spells.
In addition to this I have a class to represent a Order of magic-user known as the Order of Set. The Setites can only cast spells as rituals. However they are can combine their caster levels for a single rituals casting provided they can all cast the ritual in question. So during a siege a 5th level Setitie, 7th level Setite, and a 9th level Setite, can take ten minutes to cast as 21d6 fireball.
What I recommend if you like this idea but not the class, is to allow rituals as above with the limitation of 1/2 the highest level memorizable. And allow casters to combine their levels during the casting of a ritual only.
The advantage of this approach that it will add flavor to AD&D's magic system without requiring a re-writing of the spells. It will allow you to add an additional form of treasure, magical components rated in gp value. It also acts a natural check for the behavior of the characters as the authorities can cut off their ability to buy new components.
The downside of this approach that it will make the casting of utility spells much more common. A prepared group of magic-users can be devastating to their target. In short it will give the campaign a slightly higher feel to it's magic.
I been using this rule for three years now and found it easy to accommodate my adventure preparation to its use.