What is the penalty if you bring extra healers?

[Kreeshak]

What is the penalty in normal or heroic raids if you go with 6 healers instead of 3 or 4 or even 5? I mean like you have enough dps but more healers than you should have.

[Frozenbeef]

Not being able to kill the boss before it enrages due to less dps? :S

[Kreeshak]

I mean does the boss hit harder? Does the boss have more hit points?

Also are there any benefits of why you would want to do that on purpose?

[Jujudrood]

The penalty would be an increase in boss health and such because it scales to the size of the raid, not the roles within.
If your DPS is close to enrage timers, then this would probably push you past it.

[Sinndra]

potentially hitting soft enrages due to boss health scaling per team member.

[Notdev]

Not always just hard enrage timers, a lot of fights have "soft" enrage timers that get exponentially harder the worse your DPS is. Pretty much any fight that has adds spawn on a timer, any stacking + damage mechanics, etc.

For boss scaling it doesn't matter what role the player is as far as I know. They always add the same amount of boss HP/damage.

[Jujudrood]

I mean does the boss hit harder? Does the boss have more hit points?

Also are there any benefits of why you would want to do that on purpose?

To use Butcher as an example, with more raid members but the same DPS, you will hit 30% slower, then spend more time in that 30% soft enrage, which will still stress healers and DPS alike because of the increase in health, but not an increase in raid DPS.
If you have 2 tanks, 10 DPS, and 3 healers, he scales to 15 man. DPS all average 20k, for 200k raid DPS (excluding tanks)
Add 2 healers, but no more DPS, he scales to 17 man and has more health, yet raid DPS is still 200k (excluding tanks).