Random Game Length (RGL) appears to be thought of as a terrible game concept and helps win at all costs (WAAC) individuals abuse players looking for a good time and breaks the game. Well that's the way it seems when one reads reactions to 6th edition posts, BoLS and general opinions on tournaments, etc. The reality...it's one of the biggest balancing factors in the game and is one of the necessary controls to maintain this - it just happens to be random. Let's take a look.
Remember how we said before going first isn't always best? It certainly has its advantages but going second does as well. Going second when you know exactly how many turns are involved increases these advantages and makes some of these advantages even stronger (i.e. being able to move onto objectives last). Consider you know exactly when you have to start moving towards objectives/quarters, etc. and exactly how long you need to delay/hide units which are throw-aways/objective takers in these cases. Imagine Grey Knights with multiple psychic communions literally delaying their units for as long as possible, going 2nd and bringing them all on late game. Stupid, much like Ninja Tau but it could be annoying as hell in specific settings or be used to save one or two small scoring units for late game situations.
Now these cases can certainly be seen in normal games and you in fact see this happen. Their effectiveness? Varied. The difference? Their effectiveness is far less of a certainty. Imagine if all games always ended on Turn 6 and you could reliably and consistently bring your army in on Turns 4 and 5. You can now regularly ensure your opponent only gets a couple turns of shooting and your units can move onto objectives and control table quarters. But again, pretty poor strategy (references Ninja Tau) but what about being able to do this with just a few units such as three man Guardian Jetbike Squads or Acolyte Squads? Knowing exactly when the game ends means these guys become a lot better. This isn't a good thing as armies can then move away from 5th edition concepts such as multiple scoring units to bank on going second, etc.
This isn't the major issue in a game without RGL however and can be more or less effective for different armies (i.e. those who have reserve manipulation). The big issue is what a ton of people complain about - last turn contesting. A big part of RGL in objective (and table quarter) games is not knowing when exactly you need to be on the objectives. In 5th edition you score no points for holding an objective for six turns and losing it on the seventh but do gain a point for claiming that objective on the 7th turn. You don't however know when the game ends (though the max is of course seven) with the game possibly ending on the 5th or 6th turns. This means you must plan for the game ending then and this changes the dynamic of the game considerably.
Consider how not knowing when the game ends in the final turns affects your choices. Your opponent may have more deadly units you can target but you need to kill their Troops by the end of Turn 5 on certain objectives since you cannot contest them. If the game ends Turn 5 - awesome but if the game goes onto Turn 7, you've given the opponent's deadlier units more time to damage you and thus kill your Troops. On the other hand, you may move in to contest multiple objectives on Turn 5. If the game ends - awesome again and you'll most likely win but if the game goes on, you've potentially sacrificed shooting and have potentially exposed the unit to extra firepower. For example, zooming a Land Speeder 24" onto an objective is losing any shooting ability it has whilst bringing it within range of more guns. Again, game ends on that turn and it works out great but if the game goes on, that's firepower you've lost and increased firepower the unit is taking from the enemy.
This is what RGL brings to the game. You know the minimum the game is going to go for (five turns) and you know the maximum (seven) but you don't know exactly what turn the game ends on. This changes how you play drastically. Do you go for the objectives/easy kills/quarters early on Turn 5 and potentially hurt your strategy? Do you wait until Turn 6 or even Turn 7 and potentially lose the game if it ends earlier. You have to weigh these chances up and really prepare for a game ending on Turn 5. At this point you cannot be caught out if it ends early and work this understanding into your strategy. Without this random ending, going second would have a huge advantage and the game would become far less tactical as less decisions need to be made throughout the game but rather in just the last turn.
