First things first, all around the internet people have claimed that MSU is now dead, I however, don't think this is necessarily the case.
The aim of the game - basic concept
5 out of 6 of the missions in the rulebook are objective based. There are two ways to handle this when making a list. Either take lots of units (not necessarily small) or take a few durable units AND have sufficient hitting power to knock out enemy scorers. This is where it gets problematic, as most codexes do not allow you to have the ability to have both a lot of units and durable units.
There comes the issue that when you kit out your army with many units, they are often FORCED to be small due to points restrictions, needing other stuff in your list, etc... Meanwhile durable/strong units are often priced accordingly and so you can only take a few.
Where MSU comes in
If you've managed to crack a list which has multiple scoring units, that's durable and works for you then congratulations, for the rest of us, this is where MSU is still viable. Some people would say that in 6th edition you definitely need at least 6 troops from 1500 pts up, (and preferably more as the points increase). I'll be more conservative and say that for 1500 you definitely need 4.
When you take less than this, as soon as one unit is gone - you're effectively screwed. Also it should be taken into account that objectives can be contested/denied, so basically:
the more troops you take = the more objective-grabbing chances you get = the more likely you are to actually hold onto those objectives.
I have seen quite a few players simply take 2 larger squads of troops and then kit out their army into wall of death. The problem with this is that I don't even need to kill the whole squad. Just shoot off 25% and they're gone. This is where fearless troops come into good use, however, it is still just 2 squads. Since they're large squads, you're probably spending quite a few points on them and all you have are 2 chances to capture. In a 5 objective game, I can just let you take your 2 and then easily hold onto 3 of my own, or even just get one and contest your 2.
Now, the idea of MSU doesn't just come into play when choosing your troops. In general it is more flexible to break your army up into more units. Ever had that moment when you've poured too much firepower into a unit? Well splitting your army up into smaller "chunks" means that you have more control over where the shots go. Rather than shooting all 10 men into the same thing. You can shoot a squad of 5 into the same target, see if it's been killed and if not be able to save another 5 men for shooting something else. you are also presenting more targets to your opponent, so they have more (and hopefully more difficult) choices to make
However, there are 3 distinct problems with this:
- Smaller units die faster/are easier to kill
- Smaller units mean you have more kill points available
- First blood can be very easy for your opponent
To round off, I'm not saying that everyone should have msu in their lists, but more that it's fine to have more smaller units in your army than people suggest at the moment. Splitting a unit into two where you could have one could be a lot better, especially if it's the only 'fragile' thing in your army, so it'll be easier to keep safe if your opponent gets turn 1.
Overall, this is also a good argument for combat squadding space marines, especially as we now have the added flexibility of choosing to do so out of our transports.