My initial post on this topic was mainly out of emotion rather than rational thinking, but now that I've had a chance to mull over the situation, my stance remains the same. After giving it more thought and discussing it further with a friend, I still feel adding pets and mounts as a bonus reward for using an Elder Charm is not the right direction. Flasks, pots, and other raid specific consumables/necessities would be more appropriate and be less of a potential upset.
A specific pet per specific boss would be absolutely unfair, and I don't think this is how the system would work. If anything, all bosses within an instance would share the same "Elder Charm loot table", so by using an extra roll after any of the encounters, you'd have a shot at the pet.
The biggest concern with this model are the mounts. Mounts normally drop off of specific bosses. Would they suddenly become available from every boss in the raid instance (via only Elder Charm rolls)? That would give players 4 chances (3 charms per week plus the actual boss that drops the mount) per week. Wouldn't that be going against the "rarity" design?
If the mounts remain boss specific (but you get an extra shot on that boss by using an Elder Charm), doesn't that seem unfair to mount collectors? Pet collectors will have a statistically higher chance simply because there are more bosses to roll on, while mount collectors will only have the one boss and one extra roll.
This is such unnecessary drama that can be avoided by just leaving pets and mounts out of the equation. Leave things as they are, and find another goodie to add to the Elder Charm rolls.
Heck, if not raid consumables, I'd LOVE a 28 slot bag. Those are very expensive and hard to come by, even if they're craftable. That would be a great reward for all the effort and time spent gathering those charms.
Because the whole point of adding more to the Elder Charm loot table is to make it feel worthwhile right?
That brings me to a second issue with adding pets and mounts to the Elder Charm roll. Will they be old, pre-existing companions/mounts? Or a totally new one, made specifically for the extra roll loot table?
If it's the former situation, it doesn't exactly make the roll worth it for veteran players and collectors. Receiving something common like a Silver Tabby Cat after using your bonus roll is a bit of a let down for most IMO. Plus since many of the pre-MoP pets are readily available these days, many players will likely have them already and may not be able to profit or use it to their advantage.
Placing older mounts would probably be more well accepted since most mounts are harder to come by than some pets. But of course, then the pet collectors would get the shaft in this situation, unless the older pets that are added are the harder to obtain ones (such as the Hyacinth Macaw).
Now, if the pet/mount is a totally new one, just for the Elder Charm rolls, that would definitely feel like a reward. Everyone loves shiny new things.
But of course, the downside to this is that players would feel compelled to use their rolls just for a shot at the new pet/mount, even if they didn't need anything from that particular raid zone anymore. This goes back to my original concern where raiding collectors would then have to choose between saving rolls for progression or for their collection.
After a while and after content's no longer current, this won't be an issue. But as it stands, it being so early in the expansion, collectors will feel the pressure to choose to between saving their coins for current upgrades or using them on (relatively) older instances.
This is of course, if pets/mounts are specific to a raid zone. Example: An Elder Charm roll in Mogu'shan Vaults would yield only pet A, while an Elder Charm roll in Heart of Fear would yield only pet B.
If the rolls can yield the same pet/mount anywhere, there's no choice involved since you'll be using your charms anyway in hopes for a piece of gear.
This will be frustrating for pet collectors, though. Nevermind the RNG that will be involved, if they don't raid, how will they even have a chance? Not having access to a new pet that's only available through Elder Charm rolls (and only if you don't receive any piece of gear) will be devastating for some. It will be like the PVP vs. pet collecting scenario all over again, and many will feel obligated to just bite the bullet and do it despite their lack of desire to step into that world.
All in all, this is just the tip of the iceberg of all the potential headaches that will come with adding mounts and companion pets to the extra roll. Prevent the fire before it starts, and consider other possibilities; raid consumables, larger bags, and items that are hard to acquire but would be very useful, helpful and beneficial to everyone.
Pets are awesome, but they can't be the "fix" for everything. Honestly, it feels pretty cheap to just tack on a pet after the fact, as if pets and mounts are second-best items and used merely as compensation for poor rewards. (A pet should be a reward in itself, like the vanilla raid boss dropped pets in 5.1! :D)
Again, I know that nothing's been decided yet and adding pets/mounts to the rolls was just an idea/example of what they could do to make Elder Charms feel more engaging/fun. There aren't any concrete details on how this would even work, should they choose this as their ultimate design.
At the same time, I hope they seriously consider and choose other ways rather than just throwing vanity items around as a second thought.
All I can do now is just wait and see, and hope for the best but prepare for the worst.