I have found that they can cause confusion rather than help, especially when both stations are having a difficult time hearing each other. As a general rule M's and T's should not be used. Having said all the above about 2 minute sequences, you will find that a lot of random activity is conducted using one minute sequences. In fact, nearly all random QSO's made by W5UN are one minute sequence based. The reasoning here is that if stations are hearing each other well enough to copy random calls, then the additional time is not needed to get the required information passed.
Random activity is quite a challenge for the smaller equipped station.
However, by answering the CQ's of larger stations who are often transmitting on the bottom 30 khz of 2 meters, it is possible to make QSO's this way. It is a known fact that the signal level of a scheduled station can be several db less than that of the weakest identifiable random calling station, and still be worked. One reason for this is that a listening operator will mentally fill in the missing parts of a very weak calling station because he knows what the call is already. For this reason, very small equipped stations are urged to make advanced schedules whenever possible. However, don't give up on answering random cq's, because you never know unless you try.