Eldritch Lord said:
Should that rogue be prevented from trying the trip/immobilize maneuver multiple times because you believe it is "boring" to re-use a maneuver in the same combat? Not at all. If there is ever the circumstance that using a particular maneuver is a good idea, there will always be circumstances, however situational, where using that maneuver multiple times is a good idea and would be no more boring than a normal attack because that maneuver is the logical choice in that scenario. In such a circumstance, one should not be prevented from doing so because of the potential for spamming; spamming should be prevented in a different way, i.e. not introducing a feat/power/etc. that makes that maneuver better than a normal attack!
Yes, absolutely.
Because your example doesn't work. The rogue kills goblin 1 and then trips Goblin 2. Goblins 3 and 4 run away and you don't get the chance to trip them. Why are they standing around waiting for you to trip them?
OTOH, if I have an encounter, or perhaps a daily power, that is considerably more powerful than a single trip attempt, I can trip targets in an area attack effect, tripping the entire lot of them in a single round.
But, if I do that, and then do it again, and then do it again, that's boring as hell.
/edit to add
The basic point here is that some attacks
should be more powerful. Not every attack that the character attempts should be the same power. That's what we had in 3e and earlier. And, you get exactly that - the non-caster characters do the exact same thing, over and over and over again, round after round, level after level, with the occasional bright spot where they got to do something funky.
All because no attack option could ever be better than a basic, vanilla attack.
3e went some ways towards mixing things up - they added decent mechanical resolutions for various common maneuvers. However, the problem here is that the common maneuvers become better than a regular attack, thus, you simply shift from making a standard attack all the time to making a trip attack all the time. And, of course, this generally only applied to fighters because they were the only ones who could afford the feats to be able to spam special maneuvers.
It was an improvement over what came before, but it didn't go far enough.
4e says, as Pemerton rightly points out, "Look, some attacks are just flat out better than other attacks. This power here? It does twice as much damage as your regular attack. But, we can't really let you spam that because then, well, you don't need a regular attack. So, we make it an encounter power."
Like Pemerton points out, it's empowering players to be able to decide when they want to be awesome. Or at least cool.

If we go back to the idea that no attack can ever be better than a basic attack, but, you can do it all day long, we go back to the idea that fighter types basically do one thing most of the time, level after level.
Hey, if you like that, cool. Me, I don't. I don't want my moment of cool to be dependent on random chance. I want to be able to set it up. Now, I might fail, and that's fair enough. In the last session, I set up a nice little chain of powers with my human warlock, that utterly failed due to poor rolls. C'est la vie. But, the attempt was still under my control, not the DM nor random chance.