Actually, if you notice, you will get slight discounts on things like flights, general horde stuffs.

If you get honor for the horde, all it means is that you get honor for all 3 horde factions.

Example:
Gaining 100 faction with the horde means simply:
+100 to Trolls
+100 to Orgrimmar
+100 to Thunderbluff

Its as simple as that.

well i just hit revered with Horde = 70 priest

my stats are

Darkspear trolles  6638/21000  Revered
Orgrimmar          999/1000    Exalted
Silvermooncity      4695/21000  Revered
Thunder Bluff      4011/21000  Revered
Undercity          999/1000    Exalted

there are 2 ways i found easy to get it up 

either  1)  hand in runecloth to the dude in each major city   
        2)  Farm AV  and hand in all the blood and scraps u can get go to the Frostwolf base cash it all in ....might piss a couple people off ....spesh as i could be healing .....but na ....exalted all the way  here i come - like basically sit at the back of all the action ...then run round like mad Ninja looting ...as soon as alliance cap the GY just north of the FW towers ...use ur trinket and hearth down (so u dont spawn in the middle of all the alliance) and start handing in)

for the blood its like 75 rep for org per 5  and u get like 12-15 per for each of the other factions .....being undead i was Exalted UC before 50    *pats back* *hates braggers :/* 
but  per av    100 blood = 1500 org rep  all the scraps and meat bits = a good 2-3k org rep  giving about 1-2k rep for the other factions ..

HORDE FTW!!!

i'm exalted with the horde now, so exalted with all 5 cities.
nothing happened :-(  no cool stuff i can get, or a cool name that i can attach before my name :-( all those quests for nothing lol ;-) well i can fly cheaper now from city to city lol


edit: 1 september 2008.

in the lich king, u will get a titel: Ambassador "name"
if u get exalted with all 5 cities. i'm so happy now :-)
finally something hehe

Actually. The Horde rep bar is seperate from the Horde faction bars and cannot be checked at this time. I found this out after asking a GM about it after I became honored with the horde even though I had been exaulted with all factions for quite a while. (I loved AV turn ins) You usually get horde rep from epic quests like ones involving raid bosses or epic dungeon bosses. There are many lesser quests that yield horde rep as well. Such as the lunar festival elder quests. Even if you are exaulted with all horde factions you can still do these quests for horde rep. The same GM that told me there was a seperate bar for the horde told me that if you manage to get it exaulted that Thrall would give up his throne to and greet you whenever you where in his presence. (Same with Sylvanas and the TB and now Silvermoon leaders) They obvioulsy intended to release this bar in your faction list after the final expansion is released. So till then I recomend that you do every epic quest there is and talk to every elder annualy for the lunar festival. Sounds like fun...
good luck.

There is actually a way to see your Horde rep, but it involves a little work. 

First, open your factions window and count down each line in it until you get to the "Horde" entry.  Note, be sure to count the faction group titles as well.  For example, on my rep window I counted #1 for the "Classic" group title, #2 for "Argent Dawn", and so on.  I counted down to the "Horde" header, which was the twelfth item down for me.

Next, type the following script:

/run SetWatchedFactionIndex(12)

Be sure to replace 12 with whatever you counted earlier.

This will cause your rep bar above your xp bar to show Horde faction.  It's interesting to note that my Horde faction is currently 1099/21000 Revered, although I'm 999/1000 Exalted with all the individual factions within the Horde.

You can also use this trick to see faction for Steamwheedle Cartel and Shattrath City, as well as Horde Expedition.  None of the other faction groups gain reputation, so they will always show 0/3000.