This study is actually more along the lines of what I said...
http://www.drugscience.org/Petition/C2D.html
""A study by Azorlosa's team in 1995 addressed the effects of puff volume and breathhold duration on plasma levels. Changes in puff volume produced significant dose-related changes in plasma, carbon monoxide, and subjective effects whereas changes in breathhold duration changed plasma levels but neither carbon monoxide nor subjective effects. (50) The increases in plasma THC levels from breathholding are not statistically significant except at the lowest and highest doses."
"Thus, the plasma THC data suggest that the stereotypic behavior of marijuana smoking is useful for maximizing absorption; however, our study suggests there are diminishing returns with longer breathhold durations."(51)
"As Tashkin determined in 1991, longer breathhold durations increase exposure to tar and carbon monoxide. The diminishing returns Azorlosa has established provides a basis for persuading marijuana smokers to alter their smoking patterns to reduce the harmfulness produced by prolonged breathhold durations. This is additional evidence that the harmful effects of the tars and carbon monoxide can be reduced through use of greater filtration and change in smoking techniques. ""
I didn't find a study actually saying that it's not good to hold it in for long, but it is common knowledge (for some reason every does think that) and if you Google search, most of what you find says exactly what I said before.
https://www.google.com/#hl=en&safe=off&tbo=d&sclient=psy-ab&q=thc+absortion+seconds&oq=thc+absortion+seconds&gs_l=serp.3..0i8i13i30.12046.12263.1.12373.2.2.0.0.0.0.210.309.1j0j1.2.0.les%3B..0.0...1c.1.YdJqDKBaMP0&psj=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&fp=cbc63a80511ffcda&bpcl=39650382&biw=830&bih=487
I think people read these studies and believe it's not worth the tar and carbon monoxide vs THC, since there is less and less and it comes in your bloodstream slower and slower as your puff of smoke runs out of THC.