Their goal is to offer the same raid experience at different difficulty levels. Adding more bosses would defeat that purpose.

Adding bosses in normal or heroic defeats the purpose of RF. The whole intent was to allow more people to see content. It's counter-intuitive to then add content that you can't see.

Molten front Dailies /sigh
I hate them. I really do. However I want to buy some of the pets and goodies the vendors sell. Should I be allowed to buy those without putting in the time to do the dailies? I don't think so. I have not done the dailies and therefor I do not have the pets. I don't feel cheated, I don't think Blizzard should make it so I only have to do the dailies once each and then let me buy the pets. No it would not be right.
That is the mentality I have with the bosses. Sure i would most likely see the normal mode boss and miss the hard mode boss, but so what? If I wanted to put in the time and effort to see the hard mode boss I am sure I could. I simply can not and would not put the time needed to be a hard mode raider. However, I believe people who put in more time and effort in to raiding should be giving a little something nice like a special boss. Much the same way people who do all the molten front dailies get to buy there mushroom chair and junk.
And higher Ilvl item does not count as something special or nice. maybe unique models (not reskins) would count but not +10 to stam.

I think that movie theaters should kick their patrons out before the climax of the film unless they've spent the previous six to eight weeks watching the first two acts of the movie over and over and over again. If you're not willing to put in the time, you don't deserve plot resolution.

I'm okay with HM only bosses. A lot of raids have had them, and no one complains that they need to make a normal-difficulty Sinestra or that Algalon was too hard to get to. Those are the treat hardcore raiders get to see. I'm casual to the core (I had to do some major DW dungeon crawling before I could even use the raid finder when 4.3 dropped) and I have no problem with this.

Notice, though, that almost all of the "heroic only, super hard" bosses were in a middle or early-on tier. That means that people can come back and still experience the boss fight after they outgear it.

I'll never understand the mentality that causes someone to wish that strangers get less content in a video game than they do. It must be exhausting caring about such meaningless things in life...

I get better gear, an ach, and usually a mount. Seems pretty fair to me. Let the causal players see the content.

You guys say that LFR raiders should get something less, which we do, the ilvl of the gear is lower than normal and heroic, we also do not get the geodes, or essence of destruction.

I'm not a raider but I sort of agree with the OP, at least for Heroic versions of raids. The whole point of achievements is, as Blizzard themselves put it, "bragging rights". To have a spare boss in Heroic dungeons that has little or no lore value and gives an achievement and/or title would be harmless. It would be no different than a non-raider going after Loremaster in my opinion. The implementation of titles and achievements is, after all, Blizzard's way of catering to the basic human desire to display one's pride in having accomplished something. I say give the raiders something to show others that they are dedicated to their particular field of gameplay. Being a borderline OCD completist I've had to come to terms with the fact that I cannot possibly obtain or achieve everything in this game and you know what? That's okay. That fact is part of what gives WoW it's grand scope.