Its up to the Data Controller to govern sharing of your data
its really naive to expect your MP to open every piece of correspondence or email and personally respond.
like the police, your doctors HR or whoever that are meant to be treating your correspondence confidentially, there is no reason why they can't share that information amongst their trusted colleagues & staff members in order to provide their service to you. While the MP's have said they share their passwords amongst their staff, don't forget the information is still held on password protected systems. Someone walking in off the street can't just open the machine and access the data. If the machine was stolen, without a password just powering the machine on won't reveal its contents, and no I have no idea if the data is encrypted at rest but given how easy bit-locker is to enable it all should be encrypted now.
I share the sky PIN with my Mrs and others that stay.
I share my wifi password with visitors
I share bits of my password with strangers on the phone when I phone them and they ask me
I share my Credit card details when i phone for a takeaway
I know parents and friends pins & passwords for their phones and PC's from when they ask me to fix stuff for them.
At work we do have shared last resort 1 time passwords (of course they are changed often) as per pci/dss/sox procedures.
At work I only know my personal passwords not those of colleagues.
it looks like many commentators here are fixated by the need for a confidential password and don't actually understand what the password is there for.
For the MP's, the password is really there to safeguard the information from being accessed by unauthorised individuals. The staff are authorised by the MP to look at the MP's information. Authorisation given by shareing of teh MP's password.
Yes a better system of sharing the MP's info between staff should be implemented, a system that ensures accountability for individuals accessing the info, but at the moment the system is no worse than the MP printing everything off and shoving it in a filing cabinet in the office where all with access to the office (yes including maintenance, cleaners and security) could rifle through it, in fact its better than that.