It all depends on the airline in question.
Your experience may be distorted by that fact that (judging by your other posts) you fly legacy carriers such as BA where their aircraft are equipped with crew rest facilities. A 747 will have bunks for crew because that aircaft is utilised for other longer routes where horizontal rest is mandatory. On a JKF, for example, they will be allocated a rest break (no where near the 4-5 hours as you mentioned) and they have the option to take it in a bunk.
Bear in mind also, that there are further duty periods on either side of the 7 hour flight time (90 mins prior to departure and 45 minutes after arrival). At LHR it is also often the case that crew are waiting for up to 90 mins for transport back to base. There are fatigue issues to consider when a crew has operated a nightflight, crossing time zones and then has to drive home after the flight. This has had fatal consequences over the years.
On BA, if a crew is operating a 7 hour flight, then generally no, they would not join another flight. This really is due to how the route network is set up. There are exceptions to this, Bahrain-Doha and Abu Dhabi-Muscat being two examples. Zoom Airlines is another carrier where crew will operate a flight to Bermuda and then carry on to JKF.
On a charter airlines, you will find that crew will work harder and for longer with no rest period. For example a crew can operate a flight to Antigua and then carry on to St.Lucia. Charter airlines will generally not have areas for crew to "disappear" - neither seats nor bunks. Another factor to consider is that these airlines operate on a minimum crew basis, so it would be unsafe for groups of crew to go "on rest".
Having flown for both I can honestly say that sometimes, it is actually better to have crew out of the galley and out of the way resting when there is little to do, especially on night flight. More crew = more noise, which will disturb sleeping passengers. Cabin crew are meant to regularly offer water in the cabin during the down times, so as long as regular patrols are made of the cabin (with water) then all safety and service requirements are met. Carriers like BA have crew compliments to enable rest periods to be taken safely without affecting the service.
Summing up, I guess there is an argument that a rested crew is a safer crew. As long as you are getting the service you paid for, does it really affect you?