Indeed Akira, if one searches through the literature, there is extensive evidence for progesterone's beneficial effects on the body and for contribution to breast growth.
Here just a few more.
http://www.ncbi.nlm.nih.gov/pubmed/7640409"These data indicate that the proliferative stimulus of the mammary lobule is more intense during the luteal phase of the normal cycle and may probably be due to a synergistic action between progesterone and estradiol."
http://www.ncbi.nlm.nih.gov/pubmed/8879685"The breast tissue from women operated on in the luteal menstrual cycle phase (day 15-28 among oral contraceptive (OC) users) had significantly higher proliferative rate than breast tissue removed from women in the follicular phase (day 1-14) (p = 0.01)."
CLIMACTERIC 2005;8(Suppl 1):3–63
Pharmacology of estrogens and progestogens: influence of different routes of administration"In the breast of primates, progestogens may reduce the expression of the ERa and PR, but the estrogen-induced proliferation of the mammary epithelium is not inhibited, but enhanced by progestogens203."
"In contrast to the endometrium, progesterone and most synthetic progestins enhance the proliferative
effect of estrogens on breast epithelium."
"the mitosis rate in breast cancers was observed to be higher in the luteal phase than in the follicular phase325 »
Which, by the way, does not imply that progesterone contributes to breast cancer since it just states that there is more cell division in breasts with cancerous cells. Whether the cell division involves cancerous cells or not is another story. That's why I also tend to believe that it is best to keep levels constant as fluctuations in a woman's menstrual cycle end up resulting in constant cell death and division, increasing risk, possibly (not a fact), of breast cancer.
To date, there have been no studies, to my knowledge (but, of course, I'm very limited in my knowledge) that have shown a link between bio-identical progesterone and increased breast cancer incidence. But, if there are such studies, please bring them to our attention. We want facts.

Or maybe in the future, we will find something...but to date, no association as opposed to androgenic progestins like medroxyprogesterone acetate.
AUTHOR Shyamala G
Division of Life Sciences, Lawrence Berkeley
National Laboratory, University of California,
SOURCE J Mammary Gland Biol Neoplasia 1999
ABSTRACT:
Progesterone was identified as a mammogenic hormone several years ago
but until now its role has been obscure. Recently the role of
progesterone signaling in mammary development is becoming more clear.
The most significant observations to emerge from these studies are
(1) progesterone receptors are essential for lobuloalveolar [but
not ductal] morphogenesis.
TITLE: Interaction estradiol-progesterone au niveau des cellules
mammaires humaines normales et pathologiques.
[Estradiol-progesterone interaction in normal and pathological human
breast cells]
AUTHORS: Mauvais-Jarvis P; Kuttenn F; Gompel A; Malet C; Fournier S
SOURCE: Ann Endocrinol (Paris) 1986;47(3):179-87
ABSTRACT
adequate cell differentiation can be obtained with the successive
and synergistic action of estradiol (E2) and progesterone (P),
essentially because the progesterone receptor (PR) synthesis
implicates the previous action of E2
via its E2 receptor (ER). In normal breast, E2 stimulates the growth
of the ductal system whereas the development of acini depends on P
secretion.
TITLE: Effects de la progesterone et des progestatifs sur la glande
mammaire.
[Effects of progesterone and progestational hormones on the mammary
gland]
AUTHORS: Gorins A; Denis C
SOURCE: Arch Anat Cytol Pathol 1995;43(1-2):28-35
ABSTRACT:
The growth of the mammary gland during the active genital period
depends on a delicate balance between the action of the two major
female sex steroid hormones, estradiol and progesterone. The
regulation of growth and maturation of the gland primarily depends on the combined action of estradiol and progesterone.
TITLE Endocrine therapy of transsexualism and potential
complications of long-term treatment.AUTHOR Futterweit, Walter
SOURCE Archives of Sexual Behavior. Date: 04-05/1998
ABSTRACT:
"the addition of a progestin is indicated initially or several months following onset of estrogen therapy which may have an additional effect on breast enhancement."
This statement, made by an
expert who treats, as part of team in Germany (I believe), transsexual women and has done so for several years.
And there are so many more like this...