Recruit A Friend, worth it or just a waste of money?

I've been leveling a blood elf paladin with my brother's multi-box accounts. So far the leveling bonus has been nice (200), we've been mostly running instances and rarely do any quests. Here's the breakdown:

Level(s) Instance(s)
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15-18 Ragefire Chasm
19-22 Wailing Caverns
22-28 Shadowfang Keep & Blackfathom Deeps
29-31 Scarlet Monastery: Graveyard
32-34 Scarlet Monastery: Library
35-37 Scarlet Monastery: Armory
38-40 Scarlet Monastery: Cathedral

All 4 characters are shaman and 3 of them are getting 300% XP, which makes leveling go very quickly (1 week playing - 2 hrs on weekdays and 3-4 hrs on weekends). His first shaman and my paladin are only getting 200% XP so we're leveling a bit slower but still faster than playing by yourself. Also, handing in quests are so damn great, 17,400 xp for handing in Into the Scarlet Monastery ! Now I have a nice tanking sword.

Once my brother got his macros down and figured out how to control his 4 characters everything went great. My brother wasn't familiar with this whole multi-box thing so that's why we ran a few dungeons at a higher level than required.

Hopefully we'll get some better gear now that we've hit level 40, plate armor for me and chain armor for my brother's shaman.

It's been fun so far, but what about after the free 30 days are up? Is having 4 separate subscriptions worth the fun? Plus the upcoming expansion Wrath of the Lich King is going to be available later this year... Some people have commented that it's just a money ploy by Blizzard, because of slumping sales & to get more people playing their game instead of Warhammer: Age of Reckoning (which is out at the same time as WoTLK).

Yet some people say that WoW is just a hobby and there are other hobbies that cost alot more.