I've seen fights where the rogue was denied the ability to deal sneak attack damage, or to use most of his useful powers, for multiple rounds at a stretch due to be being slowed. I've seen fights where defenders were basically kept from "defending" at all, because the enemies were easily able to stay away from them while getting to the "squishier" targets. This is especially true with artillery and controller creatures, but even melee-focused creatures can take advantage of it.
And while you dismiss "keeping creatures from running away" as a minor advantage, it means that the entire party can more easily gang up on a single target and take it down fast.
You keep saying "People should learn to carry ranged weapons," but for the bulk of melee-oriented classes, that still leaves them unable to use the overwhelming majority of their powers. Sure, doing a basic attack for four rounds straight is better than nothing, but only marginally.
It's really not hard, unless the battlefield is relatively confined, for a creature with halfway decent speed to keep more than four squares distant from a slowed character. And the fact that a slowed character can still do something doesn't remotely alter the fact that their options (and, in many cases, access to powers) is substantially reduced--which is, ultimately, the entire point of tactical use of conditions.
I'm still not buying it. Your examples are not concrete enough to be persuasive.
Which Rogue powers were prevented? Why weren't other PCs able to help out the Rogue gain Combat Advantage? Why were monsters so far away that they could run circles around the Defender, but the other PCs could not get close enough to the Defender that melee foes would also have to get close to the Defender?
So what if one creature can stay away? That doesn't mean that all creatures can stay away.
If all 5 PCs are Slowed or if there are additional conditions on the Slowed PC, sure.
But, against one or two PCs at a time, Slowed is a nuisance.
Are you playing the game on a massively large outdoor arena were the artillery can keep backing up forever so that one Slowed PC cannot reach them? How is it that if the Slowed PC cannot run at 8 per round and get back in a group of foes?
How come the PCs cannot go back the way they came and head around a corner and force Artillery and Controllers to either come to them or flee?
How come the non-Slowed PCs cannot gang up on Artillery and Controllers while the Slowed PCs concentrates on melee foes?
Sorry, but your "easily able to stay away from them" claim doesn't make sense at all. Sure, there could be special unusual terrain where there are pits and the PCs cannot move in straight lines once in a blue moon. Granted.
But for most encounters, the Slowed melee PC just moves up to the nearest melee foe. He doesn't bother with the ranged Artillery or Controllers. He lets his allies handle those. The Slowed range PC doesn't care and just attacks whomever is within 10 or 20. Usually, not all foes are 10+ (or 12+) squares away in an encounter for most of the encounter.
Slowed could present some real issues for a Solo monster, but not much of an issue when there is a group of monsters or a group of PCs. I cannot remember a single encounter where Slowed made a significant difference. Maybe our players take more precautions. 4 out of 6 PCs can teleport. All of them are Paragon and 2 of them have an additional move action if they use an Action Point. Some also have items like Boots of Eagerness. All of them have ranged attacks.
The only things that bother my players are Dominated, Stunned, Blinded, and to a lesser extent, Weakened. There was a monster in Scepter Tower of Spellgard that made 3 out of 6 and eventually 5 out of 6 unconscious for the entire encounter once and that was scary for a while.
But, Slowed or Immobilized or even Restrained? Not a real big worry for them, even when they were in heroic levels.
It does sound like the players that you are talking about are not using good anti-Slow tactics. It's really not that hard to do so, especially if the players work as a team.
If the Fighter is Immobilized or Slowed, Slide a foe 3 squares towards the Fighter if you have a Slide power. There are a lot of ways to skin a cat, especially in 4E where every PC has a plethora of abilities. The players just have to figure out cool and sometimes different ways to use them, and they have to help each other out.
Does that mean that the players have to sometimes not use the power they had originally planned to use and use a different power instead in order to help out the team? Sure it does. But, that's typically not game breaking and that happens even when we are not talking about Slowed.