As an ephemeraologist, one of the questions I get asked the most frequently is, "How can you bring yourself to use these old items?"
It's a good question, and it's not as cut-and-dry as you think. I LOVE vintage ephemera but there are instances where I just can't bring myself to use certain pieces. More on that in a bit.
I belong to the Ephemera Society of America (ESA) (see my article here reiterating my point!) and their main focus is to preserve our history through paper. I have a feeling that most of the members would cringe if they saw me using old paper in my collages; indeed, I have gotten comments that have hinted at that. Here's my take on the subject: the paper that I use in my work is by no means rare. Some may argue that it may not be now, but in the future it'll be harder and harder to find unused telegram booklets, for example, or WWII ration stamps. This may be true.
To combat that problem, I always try and purchase a whole bunch of an item if I can afford it. I have enough vintage trading stamps to last me until Rapture. Unless I do an enormous installation piece, I can't imagine that I'll ever use them all in my lifetime. I'm 42 and I don't have any kids; where is my stuff going to go after that, anyway? I have three nieces and a nephew and I doubt that any of them are going to want my collection, save for a few neat pieces to remember me by. I suppose I could will my collection to the ESA, if they'd like it, or other collectors could grab some of my stuff at my estate sale when the time comes. But what if no one takes it - will it just get thrown out anyway?
Sorry - I didn't mean to get maudlin! :D What I'm trying to say is that when it comes to my artwork, I take an existential stance - I'm going to live for today, because I don't know what's going to happen to my collection in the future. I'd rather see my collection being used in artwork that someone else (hopefully!) enjoys than have it sitting in the bottom of a box in a musty basement.
On the flip side - if a piece has sentimental value, such as those telegrams sent to my great-grandparents - NO WAY am I going to cut them up! That seems sacriligeous, somehow, akin to spitting on my ancestors. I have a personal tie to them - their daughter is my beloved Grammie. My mom dearly loved her grandparents and would spend entire summers with them. So to use these pieces would be wrong, at least to me.