Seeing as there are Hybrid cards/mana symbols, why not simply create lands that produce hybrid mana rather than ­: Add A or B to your mana pool.?

The reason, such as there is, is that adding to your mana pool isn't currently defined in the rules. You may or may not be aware that the symbol currently has two very different meanings. When it's being paid (i.e. in costs such as ", " or the mana cost of an artifact), it means "one mana of any colour or none". The technical term in the rules for this usage is "generic mana". But when it's being added as in the ability on Quicksand, it means "one colourless mana".

In other words, in costs, is less restrictive and more useful than . But when being added, is more restrictive and less useful than .

So there's definite potential for confusion between the two meanings of . (This is why for a long time lands didn't say "add " but instead said "add one colourless mana", such as on Rath's Edge.)

Using hybrid mana symbols in the way you propose would NOT work the same way. Currently hybrid symbols are only ever used in costs: so is always less restrictive and more useful than . If you allow adding to your pool, then... that's still less restrictive and more useful than adding .

By the way, when are you expecting the player would choose what type of mana this would be used as? On Urborg Volcano, the choice is made when tapping the land. But the current reminder text you have here suggests that you'd have a floating in your pool, usable as either or potentially several spells and abilities later on in the same main phase. It seems like you're effectively proposing adding five new types of mana that could sit in pools, and I don't think that's worth it for an Urborg Volcano almost-reprint.

(It turns out the rules do actually say what happens if gets added to your pool: you choose one half of the symbol at the moment it's added to your pool, as per rule 106.8. This is for Charmed Pendant and Elemental Resonance.)

Given that - rule 106.8 - how does this wording work ?

It does work. I still don't think it adds much over Urborg Volcano, but it does work. If I had to word it, I'd phrase it as "(Add your choice of or to your mana pool.)" No need to use newbie-scaring terms like "resolves" on a simple land.

Is it, in fact, at all different from Urborg Volcano?

Functionally no, it is no different from Urborg Volcano but when has that ever stopped WOTC ? Grissly Bears, Runeclaw Bears, etc They have a long history of Functional reprints (same card, new name)

Why don't you make it add to your mana pool, but give it some other draw back, that way, it is different in function than Urborg Volcano or some of the others. It would be similar to Graven Cairns. Perhaps a pain land?

Just a thought.

what? like

­, : Add make it a 1 for 2 filter land so you tap 2 land, get 2 mana ???

Which would make it closer to Shadowblood Ridge

Funeral Pyre has already been taken http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=34889

Easily solved, thanks for catching it.