I can probably recall perhaps 5 or 6 times when the line to the oiler was plugged or frozen and prevented it from adding oil. I can think of dozens of times when the oiler just malfunctioned and did not deliver oil. If all I had was the old fashioned bulb oiler with no sight glass, the machine would run to destruction. So, I agree, the base case is a constant level oiler with a built in level glass. But, most of our bearing housings were built to include a bull-eye sight glass screwed directly into the housing. If it is there, I would leave it. If it is not there, I would add one. It probably costs less than $10US. I will pay that for a back-up that I probably won't need.
When we talk about sealing up a bearing housing, we are talking about making it air tight. The seals are contacting face seals, the oiler is removed. We add an expansion chamber on the top to accommodate changes in temperature and air pressure with less chance of air being drawn in. In combination with a full synthetic oil this can drastically reduce the introduction of contaminants. We have plants within our company that have gone to 3 year oil changes with very good success. If no oil leaks, there is no need to add oil. If the housing is air tight, the oil will last a very long time.
Johnny Pellin