Usually if it's a mission that needs specialists. But I did say I haven't done Vanilla in a ling time! But you are right. I probably wouldn't.
Well, no. Or more precise: this only applies to any Camouflage Marker without cover and Mimetism (e.g. Daylamis). For all the other Markers the Weibing will try it's best on 14s or 11s whithin 32" due to Partial Cover and Mimetism and farther than 32" it'll be 11s or 8s. A Rui Shi on the other Hand, buffed with Assisted Fire (I assume Marksmanship works on Discover since Discover applies the MODs of BS Attacks), will discover stuff exactly how you said.
For me it's a question of what the utility is of the REM Racer if/when the REM goes down, or if the REM's intended mission role turns out to not be applicable once I'm at the table. The Hacker profile, for example, can still add to a hacking net's ARO threat after the REM gets blasted to smithereens, and is a specialist. The other three profiles become non-specialist LSG corner guards, and we're talking about using them in a faction with Shaolin and Kuang-Shi for DZ corner guard competition. Er, not quite. You're right that I didn't factor in Mimetism or Cover modifiers, but your math is off. Marksmanship probably doesn't apply to Discover—the Requirements say the Trooper may only benefit when "he declares a BS Attack," and Discover lacks the BS Attack label and says it's a WIP roll that applies BS Attack-applicable MODs. I think that not declaring a BS Attack (either the Common Skill, or a skill with the BS Attack label, or a skill that calls for a BS Attack roll) means Marksmanship doesn't activate. If it does apply (which would be neato) the same benefit applies to both REMs, not just the Rui Shi. Also, sensor is better than MSV2 for Discover rolls: The Rui Shi doesn't have Sensor and so it lacks the flat +6 WIP bonus to Discover; it's discovering on 14s before range/cover, and just ignores Mimetism MODs. The Weibing has Sensor so it gets the flat +6 regardless of Mimetism, so before applying range and cover, it Discovers non-Mimetic camo markers on a 20, Mimetism -3 markers on a 17, and Mimetism -6 markers on a 14. If the plan is to Discover + Shoot a lot, the Rui Shi is in fact a better choice because Discovering on a 14 or 11 and then unloading with a Marksmanship Spitfire that ignores Mimetism is...well...really good. If the plan is to use the REM as a forward observer/fast specialist/camo revealer and you aren't planning to use a Rui Shi as a gunfighter, the Weibing is a better toolbox because you have the opportunity to clear large swaths of the board, reveal camo and hidden deployment models in ZoC with no need for LoF, and then Spotlight them remotely. The more camo markers you're facing, the better the Weibing will look. Addendum: This math is only slightly worse without the REM Racer; a Sensor bot is a really handy bit of kit to have for relatively cheap, so even if you're going to go the route of a Rui Shi gunfighter, a Sensor bot is likely worth its cost and slot in the event of camo spam.
You're right. I mixed up the Sensor's WIP +6 MOD with the REMDriver's MODs. Of course the Rui Shi won't have the WIP +6 on Discover. Still no auto discover for the Weibing which was my main claim.
If discovering is what you're doing then there's the Bao for vanilla/ISS whom is 17/14 against non-mimetic, mimetic (-3), mimetic (-6) and Impersonation :p
Or for 3 extra points and a trooper slot you get a 6-4 rempres FO specialist with a repeater, sensor, two regular orders instead of one, a second wound on a corner guard, and deployable repeaters in or near your DZ. I like Bao thematically, and if I only had one trooper slot or needed to plan for infiltrators, maybe? But between the two I’m more likely to take the REM as a fast sensor/repeater/specialist. If I’m already taking a Weibing and have 12 spare points, no assault REM, and want another Regular order instead of a couple Shaolin, the REM Racer looks good to me. It’s definitely more a gimmick than an everyday flavor, but it’s an interesting toolkit for 27 points and two trooper slots.