Alts. Love them, hate them, or face addiction with them - they're out there and they're here to stay. With the changes to the badge/loot system, plus heirlooms and LFD, it's extremely easy to level up an alt and gear them out in T9 loot in no time. While this is a huge boon for guilds going through the inevitable end game bleedout, it can be a nightmare as well.
Now don't get me wrong. I have my own stable of level 80 alts, as well as a smattering of others that are in the process of leveling. They're fun to play, and I enjoy learning how the other tank/healing classes work. Yes, of all my alts I only have a single lvl 80 that is a pure DPSer. I'm sick that way.
Back to the point
My issue comes up when during a progression raid someone wants to bring this or that alt because "they don't need gear any more" for their main. Well that's great Sparky, I'm glad the RNG has worked well for you. Unfortunately there's 9/24 others who aren't in that position. Or what if it's the MT or your star healer? Very often they don't get the opportunity to ever swap out for an alt (even on alt run nights) because they're geared specifically to handle those encounters.
More to the point however, is that your output on your alt will probably not be near that of your main toon. They don't have the gear, you aren't as familiar with the rotation/positioning, etc. All of these combine to make a farm run just a little bit longer, or even cause a wipe or two. You're asking everyone else to sacrifice a little, just because you want to gear up another character.
The very rare occasion
Sometimes, I will admit, it's necessary to bring in an alt. We do it fairly regularly with our Warrior/Rogue, because he's had to switch toons to allow us to continue raiding. If he hadn't switched, we would have stalled - and so now we keep bringing him on some fights (i.e. gunship) where there's a specific piece of tank gear he needs. This however is definitely the exception, and not the rule.
Now, here's a place where I LOVE to see your alt (as long as you've got the cash). Players who know the fights, and are willing to shell out for their gear - instead of asking to be carried through for no cost. In the two weeks that we've been running GDKPs we've rolled in over 7k per person. It's not huge, but it makes me feel better about giving gear to a 2k DPS alt running around in heroic dungeon gear.
So go, and play your alts. Frolic in the joy of no pressure raiding. Spend your gold in my GDKP, and take your chances in PUGs. Just don't ask to come to my main raids. At least not without some serious cash in your pocket.