The second string is defined as: The group of players that substitute for the starting players or from which substitute players are drawn on a sports team.
By that definition, the unspoken but true rationale is that your second string players aren't as good as your first choice, but are what you have left over.
Now in the world of professional sports, the second string players are still a skill level of a few galactic magnitudes above the rest of the world and are probably not that far behind in skill from the first string.
In the World of Warcraft, the second string players, our alts, are usually really not as good as our main characters. We don't devote the time to them to get them the gear, to get them the gems, the enchants, to learn how to play one effectively.
In short, second string WoW characters are usually craptacular.
Thus, when you find a second string (aka: Alt) person that is kicking ass, taking names and doing their makeup while driving down the road on the cellphone... you wonder what their main is like!
Now, this may be because despite their main being their main and their alt being their alt, they really are better at their alt than their main. We've all encountered this... the healer that couldn't heal their way out of a paper bag, but stick them on their dps character and they're solo-killing Brutallis while offtanking Magtheridon.
On the other hand, you have people who, though dint of trying, skill, knowledge of game mechanics and no life... who are pretty damn good at any character they put their hand to.
And yes, I do mean me! :D
(No, I'm not.)
(You believed me? I'm kidding!)
(... you believed that? *snicker*)
Now, I don't delude myself into thinking Kava the rogue is in the same league as Kikidas the warlock, immolate/spell rotation questions aside!, but I do think I make an okay rogue. I do think I make an okay hunter. I do think I make an okay (insert class here)
Well, okay instead of OMFGWONDERFUL because of gear and time spent playing and researching. But I do read my stuff, I do research basic talents and builds and information on the class I'm intending on living in for the next little while.
I learn what attributes I want to emphasize, what hit rating I need to get, what talents are especially useful, what talents are a big waste of time, what pets are best to use when, what gear I should be looking for... etc.
Not everyone does that for their second string. Their second string are strictly for farming or for holding items or bank alts or for goofing around and dying on.
I suppose my point to this is... we're starting a second Karazhan run in our guild. We have only two warlocks, who also double as tanks. Kikidas/Kathe and Absitively/Posolutely. What this means is that unless we pug a warlock (or somehow recruit one or have someone make one), we're always going to be stuck in our DPS position as a warlock. In Abs/Pos' case, not a big deal, he doesn't have a second string DPS.
In my case, it's a little frustrating. I have people I want to get into Karazhan, some who are working towards that goal and will probably never get there (Kiya the hunter, we already have a ton of hunters and Kvasira the ret pally, because who would take a ret pally over another DPS.) and others who are working towards that goal and I'd like to get in there (Kava the rogue is at Kara level and I've been in there twice, eventually Karitei the mage and Karika the resto shaman. Additionally, I have Keyami the prot warrior, but she still needs a bunch of work.).
Thankfully, this week will work for me in our Wednesday/Monday run. We're splitting it - to Curator the first night, past Curator in the second evening. I was able to take Kava to the first half, no real need for a warlock... and we have a pug warlock coming in for the Monday half, which means I can still take Kava since we'll have a warlock for Illhoof.
In general though, I expect I'm going to be very frustrated with the fact that I'm going to get stuck with either my healer Kiljara, warlock Kikidas or rawr tank Kathe. Now, badges are nice, don't get me wrong and I want to take Kathe when a tank spot is available.... but dang. It would be nice to not have to be limited.
People who ONLY have one character that's Kara ready... well, they get to go. Because they don't have someone they could be otherwise. So... they get to go as that person, all the time. People who have multiple 70's who are Kara ready... we get the short end of the stick. We're stuck waiting to see what gets filled so we can fill in what's left, unless we say, "I want to go as this person.", and then we feel terrible when the spot we declare we want to take... leaves someone else out. :(
Sucks to be second string. :(