As I think this topic has established, stern thrusters are not always required on ships. They are usually used whilst manoeuvring on/off a berth, anchoring or performing other manoeuvres. Cruise ships also use them when they are at anchor and need the keep the ship in a certain position, in order to to ensure passengers/crew can embark/disembark from the sheltered side of the vessel.
I'm working within the cruising industry, but the same principles are likely to apply to other ships - just be aware that for larger vessels (e.g. container ships and tankers), the numbers quoted below may be very different. From what I am aware though...
Bow or stern thrusters cannot be used at sea, whilst sailing at speed. They are usually switched on whilst approaching a port at around 10 knots. At 4 knots the thrusters are 50% effective, and for anything above 8 knots, they have little effect on the ships course. Some ships also have systems in place to prevent the thrusters being used at too high a speed. Most new cruise ships have two or three bow thrusters, along with 2 (sometimes 4) azipods. Stern thrusters are simply not needed since the azipods perform, in part, the same function. Some slightly older cruise ships may have one or two stern thrusters, but these are rarely used - apart from anchoring when the wind is blowing.
You may wonder why these ships have multiple thrusters - why not make do with just one? Well, there are several possible reasons, but one of the main ones is that if one of your thrusters goes wrong, you have others to use as backup - this is of particular importance when performing a manoeuvre where you have little room for error. There are also some ports (again, particularly relevant to the cruising industry), where no thrusters can be used within a specific distance from the pier - in such ports, tugs must be used to help berth the ship.
James
