I have two tilts. Love em. Are they necessary? Nope. But I like being able to monitor gravity ( as
@Knightshade stated, I wouldn't trust it during fermentation, but it will show you that its going down, leveling off, where you're at for attenuation etc etc. It gives you a nice ballpark. I would never trust it for FG) and temperature, which I find to be extremely accurate. My tilt, ink bird, and the deal I have taped to the side of the fermonster always match. For gravity readings, that ballpark allows me to catch the tail end of fermentation to add my dry hop, better idea on a D rest if I'm doing one, or just raising temp to finish off an ale. I am not sure how well they perform inside stainless steel, however.
If you go that route, I do recommend linking it with a raspberry pi. The logging and charting is nice to look back to, plus you can check the spreadsheets on any device, anywhere, at anytime. The tilt has a pretty decent built in range (again, I use plastic. Not sure on SS or glass), but its great just checking progress from my MacBook in the back room, which is too far for the tilt app to connect.
Are you planning some long term storage? Because theres tons of us using plastic with no real issues. I wouldn't let a mead, wine, or long term store a beer on plastic though. But for the regular 2-3 week fermentation, it doesn't seem to be a big deal*.
*I am not a chemist.
I have seen stuff like that randomly in stores and in other peoples houses and have regularly thought, "I bet I can ferment SOMETHING in that thing..."
And as far as the original question. I love ported Fermonsters. I do have a little big mouth bubbler as well, which at this point only really gets used for mead.