I know that the mishna says that maaser sheini* can only be redeemed on coins and not something else. (See this answer.) However, this was written around 2000 years ago, and the way money works has changed drastically since then.
[Update: As @DoubleAA pointed out in a comment, the concept is actually from Devarim 14:25, where it says "וְנָתַתָּה בַּכָּסֶף". However, because the mishna (Maaser Sheini 2:6) talks about redeeming onto copper, albeit b'dieved, we see that the pasuk is lav davka silver, but referring to money. Silver coins are still preferred though.]
Back then, a coin had intrinsic value. They were made of precious metals. They were also the only real form of currency (no dollar bills, checks, bitcoins; trade was also used, but it wasn't a currency).
Nowadays, most coins do not have intrinsic value. What makes a quarter worth 25 cents? The US government says so. Really, it's just a small copper circle with a coating.
Now, let's say you have a wheat field in Israel. For the sake of argument, i'll say it grows wheat valued at 1000 shekels per year. (I have absolutely no idea how much a field grows.) Maaser sheini is around 9%**, or 90 shekels. Granted, with the current situation of the highest coin denomination being 10NIS, it's not that much trouble to bring 112.5NIS (90 + 22.5 -- the chomesh [fifth] added when redeeming maaser sheini) in coins (11 * 10NIS + 2NIS + .5NIS = 14 coins). But what if you have 10 of these fields? Are you expected to take along over 1000NIS in coins? It would be a lot more practical to redeem it onto paper money.
So is the mishna's ruling (and all who copied it) obsolete in light of current situations?
*Ma'aser Sheini -- the second tithe taken from fruit/grain/etc. that must be taken to Yerushalayim or redeemed on coins, which are then used to buy food in Yerushalayim.
**around 9% -- approximately 2% for trumah gedolah (98% left); then 9.8% of the total for maaser rishon (88.2% left); then 8.82% for maaser sheini. You're left with ~79% of what you bring in.