Can't imagine why I'd want one. If you look at the accessories page, there's a lot of stuff there that is either specific to either the Flex or Flex+, or really isn't useful. Such as a CIP ball. Since there's no bottom dump, you're going to have to siphon the cleaning solution out, or pick up the Flex and dump it out.
Same with the sight glass. I have one on my Spike CF10 but without the bottom dump....can't imagine what I'd want that for.
Wait....there is one thing. I use a second sight glass as an oxygen-free dump of dry hops in an IPA. I put a butterfly valve on the top TC port, add the sight glass to the top of that, and then the pressure manifold on top of that.
I bleed off CO2 into a blowoff jar through that setup. I close the blowoff valve, remove the pressure manifold, add the hops into the sight glass (I can get 3 ounces in there without crushing the pellets), put the pressure manifold back on, and crack the valve about 1/3 open to allow CO2 to escape again.
This purges the hops of air as excess CO2 bleeds off. So, when the time comes that want to add dry hops, all I do is open the valve all the way, working it back and forth as I gently tap the bottom of the sight glass frame with a hammer. Down go the hops, no O2, and then I close it up. I self-carb so I generally do this with about 7 points of gravity to go in fermentation, so there's some biotransformation going on (I like it), but I could also drop them when fermentation is complete.
You'd want to be sure you don't have a runaway fermentation where the krausen clogs the PRV and blowoff tubing, but since I do this usually with 5-gallon batches in a CF10, it's not an issue. If I do 10-gallon batches, I'll put a blowoff barb on top and switch out to the pressure manifold when fermentation slows.
So--this is the only thing I can think you'd do with the sight glass. Below is a pic of my hopdropper, currently awaiting a drop in a beer I brewed Friday.