I addressed this issue once in a talk I gave for the teachers at the Catholic Schools in Great Falls. At various times when celebrating Mass at the schools I was asked to do things that were not to be allowed. Again I heard things like, "Father Pastor does this here every week." I later recorded my talk, "Lex Orandi, Lex Credendi", in the parish church with a few changes to make it more generic. You can find that talk here.
I thought I'd share the following from one of the comments to Fr. Z's blog post. It contains several sources as to why we are not to change the rubrics on the whim of the priest or the parishioners.
• Sacrosanctum Concilium art. 22.3 (1963)
o Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority.
• Canon 846.1 (1983)
o The liturgical books approved by the competent authority are to be faithfully observed in the celebration of the sacraments; therefore no one on personal authority may add, remove or change anything in them.
• Catechism of the Catholic Church, 1125 (1992)
o For this reason no sacramental rite may be modified or manipulated at the will of the minister or the community. Even the supreme authority of the Church may not change the liturgy arbitrarily, but only in obedience of faith and with religious respect for the mystery of the liturgy.
• General Instruction of the Roman Missal, no. 24 (2003)
o Nevertheless, the priest must remember that he is the servant of the Sacred Liturgy and that he himself is not permitted, on his own initiative, to add, to remove, or to change anything in the celebration of Mass.
• Redemptionis Sacramentum, nos. 31, 59 (2004)
o They ought not to detract from the profound meaning of their own ministry by corrupting the liturgical celebration either through alteration or omission, or through arbitrary additions.
o The reprobated practice by which Priests, Deacons or the faithful here and there alter or vary at will the texts of the Sacred Liturgy that they are charged to pronounce, must cease. For in doing thus, they render the celebration of the Sacred Liturgy unstable, and not infrequently distort the authentic meaning of the Liturgy.