I think we see this issue from different perspective that don't touch in terms of Venn diagram, which might cause an unnecessary conflict between us. Let me elaborate on how I see it.
I didn't mean to say that it doesn't matter whether Marcus Aurelius did indeed say what is quoted. I think truth does matter above many things in life, and my orderly nature dictates I follow through to seeing correct information prevail.
I didn't mean to dismiss the importance of authorship of quotes, either, though it may have sounded like I did. While I can't confirm the source (oy vey), I heard someone talk about quotes. At one point, they said some quite profound general truth about life... and attributed it to Adolph Hitler. They quickly "corrected" themselves by saying "He didn't, actually, but for a split second you thought about it and went 'Oh, he did? Well damn'...", implying that the source of the quote is no less important than the quote itself.
A lot of profound (or seemingly profound) quotes have been misattributed to great people of history. Apparently, people are attracted to the great names and tend to assign to them what "ordinary" people (read: less known to the general public), it seems, couldn't have possibly said. One such example is a quote by Marianne Williamson, which starts:
Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure...
...and goes on for a bit. It was misattributed to Nelson Mandela, who has used it during a speech.
To know who said a message that seems profound is important, because it unveils the motives behind the saying, thus giving the listener more perspective on the meaning behind it. Hitler saying about the strength of spirit is terrifying. Mandela, Gandhi or MLK saying about the strength of spirit is inspirational.
I think the idea expressed in the quote is important enough to follow whether or not it was actually said by the Roman emperor. It matters if it wasn't, but to me, only to a limit, since I can see its profoundness in my own experience. For others... I'm torn on whether to attribute it. My point of view is this: it's a profound idea that people could use to learn, and attributing it to a great mind increases its chances of exposure significantly; declaring that we can't attribute it to a great mind, however, will most likely let it become dust in the wind, with people missing on a helpful idea that could improve their life. I don't mean to say that those in control of the information should lie to people, but the current state of affairs seems to be mostly beneficial.
On the quote itself and its meaning:
By saying "everyone's entitled to their opinion" I don't mean to say that all opinions have the same weight or are worth listening to. Although a medical doctor may not know all there is to know about medicine and physiology, I will trust them with figuring out what's wrong with my body and how to fix it considerably more likely than to a person without a medical degree. It's not about whom to listen to.
What I mean to say is this: people tend to say crazy shit they have no idea about in the daily grind. We all see only a part of the world - that which we've been to, physically or mentally. Each of us only has so much experience when dealing with things. I believe that it's everyone's right to have an opinion based on their experience. It doesn't mean that people are allowed or somehow encouraged to not learn more about the world due to this proposition: we've all seen the idiocy to which such behavior leads.
I believe that everyone speaks their mind based only on things they've experienced. It may sound like some of the most common sense common sense phrases, but it carries a deeper meaning, one that I think lil understands already, hence her agreeing with me on that in the first place. Some opinions weigh more or less depending on what you're inclined to (for example, you'd listen to a doctor with more care and interest if you're inclined to get healed, and you'd listen to a medium or a foreteller if you're inclined to feel submitted to external forces); you might listen to an Internet stranger more if they show that they possess very close perspective to yours and have managed to overcome what you're currently struggling with, even if your close friend tells you something different because from what they get to see of your struggle, what they give you is your best way out.
Most of the time, though, people say nonsensical things about stuff they have no idea about because they want to look cool. Therefore, if you let a stranger affect your vision of things without first checking whether this new perspective aligns with your current goals - say, by claiming their opinion of a film you haven't seen as your own - you're in for a whole tsunami of crushing waves from all around the area telling you that opposites A and B are both true, that white is black, that up is down... This will quickly turn into a mess - a mess that wasn't yours to begin with, one you can't feel comfortable with even a slightest bit because it's from another person's heap.
In short... One can say whatever they want. As I recognize that what one says isn't an answer but merely a perspective, I gain the power and the responsibility to sieve through what I hear and separate worthy perspectives from unworthy ones. No one knows everything, and even most-educated specialists can fail when they meet something new and unexplained, for one reason or another. It's important to keep an open mind, whomever you listen to. It doesn't exclude listening to people: the quote advises one to consider what they're listening to (see this wonderful piece of audio art by thenewgreen).
Did that clear up some things?