It's not just the farming (although that's the most practical and worthwhile reason). It's also the travel times once you have an epic flyer. I don't recall the last time I took a normal FP in Outland...by the time they circle around and stop by every other FP along the way, I've flown in a straight line to where I want to go and arrived much faster than anyone in any group I've ever been in that didn't have an epic flyer. I'm on the ground (or hovering just above it :P) and off grabbing a coffee or a snack while the rest of my group is still waiting for their rent-a-bird to arrive at it's destination, then hopping on their slowbie flying mounts for the level-40-mount-speed trip to wherever they need to be.
Getting around Shattrath is fast and easy. Most people I know without an epic flyer only use their normal flyer for short hops in hostile territory...they prefer their epic land mount for everything else because unless you're navigating lots of cliffs/walls or trying to avoid densely packed mobs, the land mount is faster. I appreciate not everyone is into rep grinds, but man...if all I ever had were Skettis/Ogri'La dailies, I'd go insane. Now (and even before the new cooking/BG/Dungeon dailies) I have more than 10 dailies available to me...I can pick and choose the ones I want to do that day without ever falling short of things to do. If one particular daily has gotten stale, I can go do a different one and still not compromise my income at all.
Think about it from another angle: heroics and badge gear can be very, very good. You can get geared for Heroics through non-Heroic 5-mans without *too* much effort. As you start to progress through Heroics, you'll start getting even better gear and every once in a while, you'll be able to make a major, major upgrade with the badges you accumulate. Over time, you'll have better gear than your Spellstrike pieces, and then what have you got to show for them except maybe 2 bank slots taken up or a couple of Void Crystals on auction? Put forward the effort now to get your epic flyer and whether you're still wiping repeatedly on Attumen or double-checking your consumables for your next attempt on Illidan, you'll still have (and get excellent use from) your epic flyer. Take it one final step: when WotLK comes out, there will no doubt be even more gold to be made...and even more gear components to spend it on. Oh ya, if your Spellstrike Gear isn't already obsolete by then, it will most likely be entirely obsolete by level 75. Come level 78, I'll still have my epic flyer and I'll be well positioned to start saving for the next enormous gold sink Blizzard puts into the expansion (whatever that might be).