When it comes to the '+2.' Again, all I can say is that my experience has been vastly different. I currently have a power called Balance of Fortune with my bard character. Until the end of my next turn, I get to roll 1d6 whenever someone attacks the target and add my roll to theirs; when the target attacks, I roll 1d6 and subtract. There have been many times when that has made a huge difference. Hitting (IMO) can often be more important than dealing damage; that +2 can potentially make the difference between delivering a disabling effect or not delivering it. I'll also add that the bonuses I give are often more than 1 or 2 points.
Yes. That's a good power.
But a huge difference? Often?
Are there really so many encounters that it has changed more than 2 rolls total in the entire encounter?
How many times has it not done a thing? On average, if 3 PCs attack this foe and you roll a 4 on all 4 rolls, that's 80% * 80% * 80% * 80% or 40% of the time (assuming 3 PC attacks which isn't always the case). 60% of the time, it does something. But 40% of the time, it does nothing significant except a few points of damage.
And that's assuming it hits. If your Bard has a 70% chance to hit, that's 42% chance of doing something and 58% chance of doing nothing at all with 3 PCs attacking. Jumping to 50/50 if 4 PCs attack that foe. The math illustrates that about half of encounters, your power should be doing virtually nothing except maybe a small amount of damage. Yes as humans, we remember when it helped saved the day, but we tend to forget when it flubbed completely. And yes, all 5 PCs can attack that one foe, but that's rarely the case. There are usually reasons for not all of the PCs to attack one foe (even with the concept of focus fire), at least if the DM challenges the players. And all 5 PCs could use an action point and attack the foe twice. The power can be made a lot more effective. But some encounters do not have elites or solos (many of which have good Will defenses), so there is a good chance that most of the time, the other PCs are not using action points with this power against a standard foe.
And to be a huge difference, it really would need to affect at least 3 of the 4+ rolls which is somewhat rare. Even affecting 2 of the 4+ would be somewhat rare and can just as easily be done with a Controller power. An Immobilize or Daze/Prone or Dominate or Stun can help the party as much or more than this does. Our 16 level Paladin tries to Dominate a foe every encounter with Siren Voice. If she hits compared to your Bard's power hitting, it's -60% chance for the foe to hit a PC (the foe doesn't attack a PC, so it's the equivalent of a huge minus to hit) and +60% chance for the foe to hit a foe. As opposed to -5% to -30% for the foe to hit a PC and a +5% to +30% times the number of PCs chance for PCs to hit a foe. To get to that same 120% swing, you have to have an average of 6 PCs attacking the foe. The Paladin could also have a foe jump off a cliff or do other things. Your power does not have that versatility.
Now consider how much your power here affects an entire encounter. One foe out of five for one round out of six or so.
Huge?
Yes, it can definitely help, but at 17th level of what all 5 PCs can do in an entire encounter, it's really a drop in the bucket. In our 16th level game, we would consider this a solid, but not great power. Helpful, but not huge the vast majority of the time. A lot more helpful against solos, but not so much in more standard encounters.
At 17th level, this power is sometimes stopping ~25 points of damage and a possible effect , and sometimes adding maybe 50 points of damage and some possible effect(s) in an encounter where the foes might have 1000 hit points. Measurable, sure. Huge? Probably not too often.
As an example in our game, our 16th level Slayer usually hits on a 3 or a 4 on the die (she has tons of bonuses to hit and charges most rounds). So, adding +6 via your Bard's power isn't really going to help that much. If she needs a 4 for a given foe, 90.8% of the time (5% she rolled a 3 and 5% * 84% she rolled a 2 and you rolled a 2 or higher), your Bard power here does absolutely nothing for her. Again, assuming your power hits (we're actually talking closer to 94% of the time, it doesn't help her).
It might help out your group more, but would do little for some of the PCs in our group. So like I said, it can be group dependent and it can be a bit illusory or misleading as to how much a given buff or debuff actually helps. All buffs and debuffs help out at some point in a campaign, but they don't necessarily help out every encounter.
At 17th level, the 16th level Cleric power Cloak of Courage is huge because it hands out 120+ temporary hit points per Encounter in a 5 PC group and since the PC Cleric can do it before an encounter even starts, it doesn't typically use up a Standard Action to use it like your power. That automatically saves the PCs a minimum of 2 healing surges per encounter. And it's dependable. There is no chance to miss with the power or for it to not help at all. Everyone except the Cleric gets double the effectiveness of the Toughness feat at 16th level for the cost of a single encounter power.
Your power would probably average just under 1 healing surge per encounter savings (by taking a foe out slightly quicker sometimes and by preventing some minor amount of PC damage and effects sometimes), possibly 2 in a solo fight with players using action points. It has the potential to be fairly awesome, but it probably isn't in most encounters. As one example, if your Bard misses with it.
But, this power here is far from making a huge difference compared to many other similar level Encounter powers in the game system. It's good, but not great.