EVE Online is a game where collective action trumps individual action. It's certainly a game that can be played solo although nowhere near as effectively as when your main's activities are supported or enabled by an alt. CCP provides 3 character slots per account, but skillpoint training can only progress on one character at a time. So where the real alt-play comes in is with dual boxing -- running two accounts at once to facilitate an activity. The main/alt duo can take a number of forms, by no means limited to the following basic examples:
Missions -- A mission-runner is paired with a dedicated salvager/looter, maximizing mission bounties and loyalty-point gain with the near-simultaneous collection of modules and salvaged ship components.
Mining -- A Hulk or Mackinaw pilot mines to a jetcan and works with a hauler, allowing for continuous asteroid and ice belt mining, thus optimized isk generation.
Exploration -- A covert ops pilot scans down complexes with a combat pilot to clear it and hack the cans, with little to no risk to that expensive yet worryingly fragile exploration setup.
Trading -- Keep an alt, or several, docked in trade hubs for price checks and buy/sell orders, as well as trade runs in an Industrial.
Piracy -- Piracy is a difficult profession to make a fortune in, but running an Empire industrial or trade alt allows a player to embrace good as well as evil, and those relatively safe isk rewards that come with carebearing.
Scouting -- Perhaps the most prevalent use for an alt is as a scout, who locates gatecamps a few jumps ahead of your main, saving a lot of frustration as well as your Tech II or faction ship and modules.
These are just a few examples of the main/alt dynamic. A much more comprehensive analysis of how alts are employed, and the paranoia this fosters, is found in a piece at Terra Nova by Nate Combs. From what I've seen in-game, and what I've done, it would seem that metagaming is actually part of the intended gameplay in EVE Online.
This notion is reinforced by CCP's periodic Power of Two offer, where you can add another paid account for a significantly reduced subscription price. Some lament that the game allows the unchecked use of alts, much less encourages it, particularly the common practice of placing alt spies in rival alliances or war-target corporations. Others embrace the metagaming aspect of EVE, with the view that the additional skillpoints and another pair of eyes, or Neutron Blasters, are tools that the devs have made available, and which should be used to maximum effect in-game.
Where do you stand on the use of alts in EVE Online? Does it foster a more varied gaming experience for the individual, or is it an avatar arms race where you're struggling to keep up?