So, say you have the option to launch a free WSO or purchase a solo ad.
Note: a free WSO, as in, an WSO that you offer for free upon opting in to your list.
If you launch a WSO, you might get 50-100 opt-ins, just as an example. This will cost $40 and those that opt-in most likely aren't in tons of mailing lists.
However, say you purchase a solo ad. The cost might be around the same ($40ish), and you might receive more opt-ins, lets say 100-150. These opt-ins however are constantly sent these messages (as expected from someone opted-in to a list in which the owner sells solo ads), and chances are they're in a good amount of lists already.
The more lists someone is opted-in to, the less likely they are to open your email and most importantly, purchase something. <-- my belief. Right, wrong, what do you think?