In my own experience I’ve found several points about a closed fit to be worth more than any sense of occlusion:
(from the 2016 review paper’s summary at end, on disadvantages of an open fit)
“Second, the interaction of the direct and amplified sounds can cause audible artifacts and can lower subjective sound quality. Third, the benefit of adaptive features such as directional microphones or noise reduction algorithms can be decreased, and the available maximum gain before feedback is reduced.”
One of the papers cited in the 2016 review is a 2006 Widex technical analysis of the effect of vent size width and length on hearing aid performance:http://content.widexpro.com/images/researchArticles/DAR84.pdf
The 2006 Widex paper covers many of the same points brought up in the 2016 review paper cited by MDB (and is one of the references for the review paper). On the most PRO side for venting is minimizing occlusion, making the sound of the user’s own voice sound pleasant to him/her. On the CON side for venting, the paper has an interesting figure showing word recognition scores drop by about 1/3 (roughly) on increasing venting, depending on what you take as the max possible word recognition score in test.
Then there is the following line from the 2006 Widex paper just to the left of Figure 4 on word recognition score as a function of vent size.
"Open-fittings reduce high frequency gain. Open fittings (or larger vent diameters), for the most part, have been used for people with a high frequency hearing loss. It should be clear from the above observations that the rationale behind this practice is to maximize “comfort” with one’s own voice and not the audibility of high frequency sounds. Indeed, an open fitting typically results in poorer high frequency audibility. The clinicians and the wearers must understand the objectives (and limitations) of open-fitting so realistic expectations can be formed. "
I found this most interesting as for me, switching from an open dome to a closed power dome, although I got the occluding effect, in return speech seems much crisper and cleaner. The 2006 paper authors also bring out the point that depending on the configuration of your ear canal, etc., there is the possibility for direct sound reaching your ear through the vents to be out-of-phase with sounds produced by the HA’s and decrease intelligibility on that basis. ~Same points as made in concluding summary for 2016 review paper cited by MDB as OP in this thread and quoted in part at the start of my post here.