I think that a lot of people here would...um, no pun intended, defend...the idea that a good defender should be able to attack while still using their buffs and heals. The "offenders" were actually far more useful in the later stages of the game than people who thought of the archetype as purely a protect-and-heal role, because by the time everyone was in their 30s many archetypes were sufficiently well protected through their own powers as to not need tons of defensive buffs. When you're a level 40 defender teaming with two tankers, a scrapper, a blaster and three controllers, your heals are not going to be needed. Your damage, on the other hand, is always welcome. 
That said, if I was reworking Vigilance I might make it into a conscious choice rather than a passive effect--that is to say, something you could choose to be an endurance discount or a damage buff, rather than something the game chose for you. Giving you the option between damage and endurance makes it more useful in different situations, instead of being something that you only wanted sometimes.
I probably didn't describe things as well as I could have. Let me give my definitions of Pure defending, hybrid and offending strategies.
In the early days of CoH (mostly due to trinity thinking) there were many defenders who skipped almost all of their blast sets. I never believed in this kind of defending, so damage was always part of my playstyle.
From about the moment I first read about repeat offenders and the strategy they employed, I kind of formed my own opinion on defending vs. offending. Later, I discerned a third layer which is kind of a hybrid. If viewed as strategy, each has a place in the sun and a very good defender can switch between those when needed.
A Pure defender is one who places first emphasis on his/her team defensive capabilities. They start to see things like if times correctly, Twilight Grasp will become an autohit (if you can get the power to hit just as the mob faceplants then it hit's all but automatically). Doing tricks like this will, by necessity, require the player to lose an opportunity to blast to make sure the team gets the correct healing it needs. Blasting comes something to do as cycles allow.
Offending is the player playing a defender as aggressively as possible. Scrapperlock becomes a trait here and the player only uses the buffs/debuffs as they need them. If a team member gets them then that's a plus. You've got things to do. As you've pointed out this is a strategy that plays very well on veteran and higher level teams.
Then there was a hybrid of the two. Delaying an aoe buff because the player was finishing off a Lt. is perfectly acceptable since you are still adding to the team as a whole. Since you are as concerned with your offensive capability you might not get each person covered with your aoe. If you are tossing out a twilight grasp, you won't really take time to pull off your trick and just rely on your normal acc to get the hit. Using flash arrow, you can't take the time to make sure it will hit every enemy because you need to get the offense going.
Each shortcut you take from pure defending gives just a bit more opportunity for your team to take damage giving you an end discount. Keeping these three strategies in mind, vigilance end discount helps the offending end more.
Even solo, the offending mindset (mandatory for solo) benefits the player cuz, well... moar damage n' stuff.
