[Jackie07]

Looks like they've made the recommendation based on some current research/report. I think the point is not to encourage 'everyone' 'blindly' take supplements of any kind. Since many of us cancer patients tend to be short of Vitamin D due to reduced outdoor activity during/after treatment, I think we should take the supplement prescribed by our doctor if the lab result shows deficiency.

PLoS One. 2012;7(5):e36617. Epub 2012 May 7.
A prospective randomized controlled trial of the effects of vitamin D supplementation on cardiovascular disease risk.
Gepner AD, Ramamurthy R, Krueger DC, Korcarz CE, Binkley N, Stein JH.
Source
University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin, USA.
Abstract
Vitamin D (VitD) supplementation has been advocated for cardiovascular risk reduction; however, supporting data are sparse. The objective of this study was to determine whether VitD supplementation reduces cardiovascular risk. Subjects in this prospective, randomized, double-blind, placebo-controlled trial of post-menopausal women with serum 25-hydroxyvitamin D concentrations >10 and <60 ng/mL were randomized to Vitamin D3 2500 IU or placebo, daily for 4 months. Primary endpoints were changes in brachial artery flow-mediated vasodilation (FMD), carotid-femoral pulse wave velocity (PWV), and aortic augmentation index (AIx). The 114 subjects were mean (standard deviation) 63.9 (3.0) years old with a 25-hydroxyvitamin D level of 31.3 (10.6) ng/mL. Low VitD (<30 ng/mL) was present in 47% and was associated with higher body-mass index, systolic blood pressure, glucose, CRP, and lower FMD (all p<0.05). After 4 months, 25-hydroxyvitamin D levels increased by 15.7 (9.3) ng/mL on vitamin D3 vs. -0.2 (6.1) ng/mL on placebo (p<0.001). There were no significant differences between groups in changes in FMD (0.3 [3.4] vs. 0.3 [2.6] %, p = 0.77), PWV (0.00 [1.06] vs. 0.05 [0.92] m/s, p = 0.65), AIx (2.7 [6.3] vs. 0.9 [5.6] %, p = 0.10), or CRP (0.3 [1.9] vs. 0.3 [4.2] mg/L, p = 0.97). Multivariable models showed no significant interactions between treatment group and low VitD status (<30 ng/mL) for changes in FMD (p = 0.65), PWV (p = 0.93), AIx (p = 0.97), or CRP (p = 0.26). In conclusion, VitD supplementation did not improve endothelial function, arterial stiffness, or inflammation. These observations do not support use of VitD supplementation to reduce cardiovascular disease risk.

Breast Cancer Res. 2011 Aug 16;13(4):217.
Vitamin D and breast cancer: interpreting current evidence.
Chlebowski RT.
Source
Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, 11243 West Carson Street, Torrance, CA 90502, USA. rowanchlebowski@gmail.com
Abstract
Preclinical investigations and selected clinical observational studies support an association between higher vitamin D intake and 25-hydroxyvitamin D levels with lower breast cancer risk. However, the recently updated report from the Institute of Medicine concluded that, for cancer and vitamin D, the evidence was 'inconsistent and insufficient to inform nutritional requirements'. Against this background, reports examining vitamin D intake, 25-hydroxyvitamin D levels and breast cancer incidence and outcome were reviewed. Current evidence supports the pursuit of several research questions but not routine 25-hydroxyvitamin D monitoring and vitamin D supplementation to reduce breast cancer incidence or improve breast cancer outcome.