[quote=@ADamnFiddle]
[@Unoedipal]

I thought it was a "mutual agreement" thing or something. At least, that is how I remember it. 

...But that was long ago. 
[/quote]

I mean, if there's a quote for that, sure. As it is now though, the Servants whose ME has been commented on are basically:

Herc: Always active (RN), mitigated slightly due to Guda(k)o not being good enough but still continuous (GO).
Lancy: Always active (Self-imposed) 
Spartacus: Always active 
Fran: Toggleable (CMIV, Apo) 
Lu Bu: Always active 
Jekyll: Nonstandard, active when NP's in use 
Caligula: Always active, exception made for when he's in areas that the Moon can't access (GO, Dantes event)