Riffles are 12% more expensive to reinforce than RE and that is not trivial, and you are also forgetting the the upkeep cost and pop limit where rifle have 40% more.
4 men RETs are incomparably inferior to rifles by more then 12% performance difference.
And while we're at forgetting stuff, RET 5th man doesn't cost 0 pop. At vet3, where RETs actually matter, the pop difference is a whooping ONE.
That 12% of reinforce difference is still only THREE compared to previous eight menpower, not really that cost effective any longer.
The are actually a good option for carrying Bazookas and (doctrinal)flamers.
RE see allot more action than Pioneer or C.E.
They USED to be a good option for carrying zookas, because where really cheap and easy to maintain, atm its not a no brainer any longer and zook rifles are just as valid option for standard equipping, if you need additional squads, then yes, RETs are better, but until vet3 will perform and survive much, much worse.
Flamers aren't really an argument at all, because no other squad can use it.
RE see a lot more action then pios or CEs, because they don't need to spend half of their time base building , mining or repairing armor.