The basis for the necessary rejection of Catholic penance in the reformation was threefold. a) We need no human priest as we are priests ourselves. b) we have access to Christ our high priest without any other mediator, and c) we never, under any condition, need to do a 'work' to obtain forgiveness - ever.
Forgiveness can only be obtained by faith in Christ. Confession of particular sins by believers is not to obtain justification or general forgiveness but particular application of our previous justification in cleansing - this is called forgiveness in brining us back into better fellowship with God. That is, as we have already been declared righteous by faith, God has sworn to give himself no other choice, according to the justice of his faithfulness, except to forgive is. If God has in a sense no choice (from his own gracious decree) how monstrous is it to imagine that we should approach a man, rather than our own high priest, to declare our forgiveness? Our forgiveness is not optional, rather it is our reasonable expectation to receive of God by faith, apart from any good work. To add to this monstrosity that we should receive an evil insinuation as though we also had to 'work' for forgiveness is a blasphemy against the high priesthood of Christ.
What does the scripture say:
Christ is our high priest who we are to approach, not St. Somebody, Mary, or any earthly priest:
For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin. Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need. (Hebrews 4:15-16, ESV)
We are all priests:
But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light. (1 Peter 2:9, ESV)
We obtain full and general forgiveness in our justification without respect to any good work:
For we hold that one is justified by faith apart from works of the law. (Romans 3:28, ESV)
As a result of full pardon we can be cleansed in particulars that bother our conscience (without any requirement of a work, of course) by simply directly confessing our sin to our only high priest, Jesus;
If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.(1 John 1:9, ESV)
Under these consideration most Protestants fully reject the need for penance as a sinful thing contrary to faith in God. Actually, anyone who has performed any kind of penance under the guidance of a Priest would technically have fairly good reason to confess that sin to Christ. One must always go straight to the high priest and not ignore him by devious means and such a show of godliness that denies the power thereof.