How do you know the opponent of yours even have that addon? couldn’t they just be really coordinated, just know what to do and happen to be real good PvPers or just happen to be fast clickers or tab cyclers? (i mean this isn’t 2004 anymore) Why it has to be only conclusion? I know that sounds “disingenuous” to examine into a problem you have and come to more then one conclusion you have and not automatically assume they have addons or cheating right off the first bat, but it really isn’t. 
Heh… automatically assume. Kinda ironic actually. 
I automatically assume your an assaultron assembled to annihilate the atrocious addons apocalypse with your astute automated accusation. 
/j 
Don’t worry, it’s an unfunny joke, i cringe myself at my own crappy humor, but it’s all for good natured hijinks. 
Maybe instead of asking to ban the addons (Since Blizzard doesn’t think this with classic) or ridicule players who does play with addons (and you know what? that’s fine to do, if you don’t want to play with them, don’t add them. This isn’t retail, reputation and server matters does it not? Though i wouldn’t go as far to kick them out of groups if you hate them or RCR them for cheating, that’s just more then petty.), maybe you can learn to embrace it or just ignore it? you know, deal with it?
What’s wrong with that? If you are good, you wouldn’t need it. If you aren’t, you only gain a slight advantage. It’s the same thing on Retail. You’re not forced to do anything really. I don’t have the power to come into your house and force addons down your throat like religion. 
In terms of a list of reasons why WoW has gone down hill, this ranks a very distant 35th since Vanilla does allow addons and even has players (granted not the majority, maybe that’s the difference maker) who play with addons. If Blizzard has a problem with addons, then they shouldn’t let the world from 16 years ago think otherwise.
Or you can make a group and make 39 friends to know them personally if they don’t have DBM and just raid MC DBMless. Hey, that’s how it’s done in Vanilla right? Make friends with 39 random people and get to know them and get them together for this one raid? Ya know… Classic? 