Unfortunately, there are people that love to flaunt the C in SCA. It means different things to different people (and this is fine), but there should still be a baseline from which we all work. The question is, where do we draw that line?
I've heard more than one person describe what we do as "The Middle Ages - as they should have been". I think that is fair. I think that the Society's rule of a "Reasonable Attempt" is also fair. It makes what we do accessible to far more people than it would otherwise be. First, it enables people to get into the hobby for a minimum amount of money and second, it allows them to see what is out there, and to learn, and enjoy it all, without worry over whether or not they used the right thread to sew their costume.
So in my estimation, that cotton-poly blend T-tunic, cut from an old bedsheet which just happens to be some obnoxious color really is FINE. That individual is trying and if the garb-police do not get to them, they will probably stick around long enough to learn more. They might even decide they want explore more in the realm of costuming and then next piece they make is out of a nice, convincing wool blend. And the one after that is hand sewn. And years down the road they are teaching classes on research and getting their Laurel.
Or maybe they never care about garb, and continue to wear that bedsheet tunic while they help out in the kitchen at events or just show up to socialize with their friends and enjoy the atmosphere. Both of those are perfectly ok as well. There are so many different people in the SCA and there really is a time and place for so many different things.
I actually got "attacked" at my first event about a belt I was wearing. I was mortified and more than a bit angry. The belt in question, was my imitation of the wrap around belts you see on a medieval bliaut (similar to this one http://www.revivalclothing.com/12th-13thcenturybliaut.aspx ), only mine was canvas encased in cream colored rayon challis (because its what I had) trimmed on both sides with a wide black trim decorated with pink flowers. I was told I could not wear a white belt (even though I would not in any way classify this as white... lol). How much better would I have felt had that person complimented my first garb efforts rather than assaulting what they perceived as an indiscretion?
Fortunately, there are not too many people out there like that (and just a note, those folks are in ALL hobbies, not just the SCA). And some of them are just so enthusiastic about what they are doing, and so eager to share what they have learned, that it can sometimes come off wrong to a bewildered newbie.
So, there are good reasons for many anachronisms (cost, time, interest level, availability of materials). I draw a line at some things (like blue jeans... yes, denim is period, but jeans, my friends are not, no matter how badly you want them to be). Is a newbie wearing jeans under a tunic to their first event? I give them a pass. Is someone working hard to set up pavilions at an event wearing jeans under a tunic? Sure, another pass. Is someone who has been in the society for 20 years wearing jeans to a 12th Night feast? I have to draw the line there. We all need to keep in mind that the atmosphere at events is part of what makes events so fun, and at least hitting that bare minimum "Reasonable Attempt" actually helps make an event more fun for all of those involved. If people are working hard to clean, set-up, cook, play music, display arts, fight, and tear-down, one can at least put in the time to go to Wal-Mart and purchase a pair of nondescript sweatpants to wear as part of their garb.
As an end note, do what you can too add to the atmosphere of an event and, please, try to be kind to that newbie in the polyester tunic, as someday they could be your King ;-)