My understanding is that monsters in 4E come with a role attached. Artillery, Brute, Striker, Minion, Solo. I was trying to design a 3.5 encounter where an ettin has a big sack full of centipedes, and I wanted to get more synergy between the ettin and the swarm. So I thought, "Can one of them act as a striker? Should I make it a bunch of individual centipedes who will go down in one hit (ie minions) instead of a swarm? Should I add some kind of artillery component for a really good encounter?"
Am I thinking about this in the right way? Does the concept of roles really help make better encounters? Or should I just think about it in terms of "the swarm attacks Fort, so I should put in a monster that attacks Will," or "the swarm hits the wizard and the ettin hits the fighter, so I should put in something that fights clerics."
Monster roles provide a guide for what the monster does in the combat encounter. Brute and soldier are the clearest IMO.
A brute is a big melee combatant with low AC, bunches of hp, and high damage with possibly low accuracy in attacks. They are dangerous up close but the PCs will hit them often.
Soldiers are high AC melee combatants who do less damage but hit often. They are hit less often and therefore last longer, though they are not the massive damage threat that brutes are in melee, so PCs can stand against them and survive getting hit for longer.
Your ettin is generally going to be a brute role, high damage melee but should be an easy target for the PCs to unload on.
The swarm has interesting aspects with certain immunities making melee tougher against them and making it dangerous to be around them. This is sort of controllerish and can force PCs to decide tactically to either suffer while near them or avoid the swarm and allow area effect PC powers to shine.
There is no right or wrong for constructing opponent makeups, the different roles simply provide different combat experiences.
For instance if you change the swarm to multiple minions then you allow everybody who hits one to shine but you take away the swarm hazard area aspect of the combat.
Adding in other combatant roles is not necessary for a good encounter, it just changes the dynamics of the encounter.
Strikers do high damage to single targets, are maneuverable, but are comparatively frail. To turn the ettin into a striker role from a brute give him less hp and a little more speed, perhaps some charging/bull rushing abilities.
To turn the centipedes into strikers make them individuals with good attacks, and good maneuverability (spring attack and high speed is pretty good). They are already comparatively frail in AC and hp so that won't need to be adjusted, though making them minions emphasizes their glass cannon frailness even more.
With a swarm the centipedes are a controller style weapon for the ettin to control the battlefield terrain conditions. With the centipedes as striker minions they are more of a shotgun blast to inflict damage.
Both can make for an interesting encounter experience.