How do we manage wireless connections without a network manager?

This excellent answer walked me through connecting to the protected wireless network at home. In short,

nano /etc/network/interfaces  # enable dhcp
service networking start
nano /etc/wpa_supplicant.conf  # indicate ssid and password
ifdown wlan0  # name determined by 'ip a show'
ip a flush wlan0
ifup wlan0
wpa_supplicant -Dnl80211 -c /root/wpa_supplicant.conf -iwlan0 -B
dhclient wlan0

Another answer suggests adding this to /etc/network/interfaces:

auto wlan0
iface wlan0 inet static
address ASSIGNED_IP
netmask 255.255.255.0
gateway THE_GATEWAY
wireless-essid YOURSSID
wireless-key WIRELESSKEY_HERE

Because over time I will be encountering different networks, with different ssdi and password, I was wondering which would be the best way to approach this, without a network manager. Priorities:

• Don't disclose ssid and passwords if my laptop is compromised - those are sensitive information that people have entrusted me with.
• Don't clutter /etc with a ton of custom(user-created) files.
• Don't use high-level utilities, like network-manager or wicd.