It is very OK. It's just a neutral. For the case of LoyceV, he's just trying to warn people that is his alt in case others might think it's an impersonator and also it's not much secure as compared to the main account since it's accessed through untrusted/devices. So no one should make any deal through it.
-"Opening multi accounts is legal" says in the forum rules. Why is this given negative? Would it be a problem unless it signed the same signature?
It's all Ok until the accounts start exchanging merit, abusing signatures, trust abuse etc
-The purpose of the multi account is: "expressing yourself more comfortably". How can I express myself more easily if I say "this account is mine"?
You can still have an alt not known to anyone. Just don't abuse merit, signatures, trust etc