For the immediate "short term", it is imperative that certain vital systems are maintained in working order. Life support (we all wish to breathe at regular intervals, and maintain a certain species-specific body temperature) is at the top of that list, along with other necessities such as food, alcohol (which often requires theft or other illegal dealings, as described elsewhere), and the holodecks. The holodecks on the Murgatroid have a higher safetey record than on any other ship in the fleet; 50% of the Murgatroid is given over to holodecks, which "keeps the crew off of the streets", as it were. The holodecks allow them to experiment, interact, and destroy in perfect safety without affecting the rest of the ship; and it is one of our highest priorities to keep them working.
In the medium term, the ship and crew must be able to deal with the aliens, anomalies, and other hazards of space that we may encounter from day-to-day. This means keeping the bridge manned with competent personnel, the sensors and shields on-line, and so forth -- Not all of our personnel are as competent as we'd like, and we do end up short from time to time (and I often find myself filling in extra bridge duty when that happens)... But, we have been fortunate so far in that regard, that we have yet to meet anything we can't handle (or run away from).
Finally, in the long term, we must (to a certain extent) follow the orders and some minimum standards of Starfleet. No matter how much we may ignore or scorn the orders of Starfleet Command, we don't want to anger then so much that they feel our existence has become more trouble than we're worth -- the rest of the fleet, after all, has us vastly outnumbered and outgunned; and we'd much rather maintain our membership in Starfleet for the protection that it brings: Romulans and other hostile aliens wouldn't think twice of looting and destroying random civilian spacecraft that they come across, but they'd all think twice about openly attacking a Starfleet ship (unless they really thought that they could get away with it).
So you see, there actually is quite a bit more organization to the Murgatroid than meets the eye -- but still, within that framework, crewmembers are given a remarkable latitude for pursuing their own interests and lifestyles. Many of the crew (those who survive the much-lower-than-fleet-average life expectancy) do not consider being on the Murgatroid to be a trial or punishment at all, and would not wish to serve anywhere else.