I guess the terms "main" and "alt" came from raiding. Your main character is the one you play with most - the one you raid with the most. It does not necessarily mean the one you leveled to 60 first.
When you start to raid you don't normally say - I'm going to raid with two characters. There usually isn't room. Guilds barely want to recruit another rogue - let alone someone with a rogue AND a hunter.
Also guilds that raid to progress, progress more slowly, if it is spent gearing up two characters instead of one - since it can't be done at the same time obviously.
Some people join guilds with their priest (because healing is in short supply) so that they can get their foot in the door. Then if allowed, they swap over to a dps class (because arguably this is more fun to play).
Now on the positive side, if you have 2 classes you can play then you can fill in here or there as the need arises. But to counter that I say, why not just recruit the needed class?
The problem comes when you have alts still wanting to go to tired-out (my opinion) instances that people with mains are sick to death (also my opinion) of going to.
People that do this say they don't have alts: "all my characters are my mains!" is the way they put it.
So you go to Molten Core AGAIN, because they want to get their 2nd or 3rd class geared up.
All the while, the guild could be beating the door down on new content.
If I had decided to keep raiding "hardcore" I probably would have had to leave my current guild, because it is very "alt-friendly". Honestly I'd much rather spend my time moving forward with the new instances, than running in place at the old ones like we choose to do.
The weirdo clash theory
11 hours ago