I might as well just give my 'two cents' on the topic of rares.
TL;DR Version: A "Small Ammount of Rares" will be hoarded, and used rarely in trade.
"If you do Decide to be insane and Read Version", here is a giant wall of text:
Some of the members that emigrated from Worldscape might recall a project called "Worldscape Blitz", which was in a ways, in-between a 2006 Remake and a normal RSPS. On the first few days of the server, rares would be released daily though drop parties. There were Crackers, H'Ween Masks, Disks of Returning, and Edible Rares (Easter Eggs, Pumpkins, Half-Jug Wines, and Disks of Returning). At first, while the Economy was still weak, the rares at first were sold for about 10k each; normal, for right when they were released.
Yet people looking back at what happened to RS, they began to stockpile on the rares. Server Population was decreasing, while inflation was increasing. Phats jumped from ~250k each to over 1m, along with prices of the other rares changing as well. I wouldn't exactly say that demand "decreased", but as the ammount of rares stagnated, prices of other items (Barrows, Bandos, BGS, SS, you get the point) fluctuated much more then the rares, they were lost as a median of trade. Basically, prices of other items rose and fell much more then rares, which was stagnant.
In my opinion, rares will tank the economy. You need a large playerbase to make discontinued items a valid portion of the economy, or else they'll hyperinflate/deflate. One example of how discontinued items are GOOD for an economy would be a game called "Team Fortress 2", where some limited edition items (For example, "buds", worth ~27 keys [rougly $67.50 if you decide to buy items worth it]), as well as Extremely-Rare items ("Unusual Hats", which can be worth INSANE ammounts of money), which help to keep the economy stable. Of course, money does guide the server, as one of the main bases for trading can be bought in the in-game store.
It's been seen time and time again, that having discontinued items with the intent of hoarding will harm the economy. Take a look at the "Fish Masks" that were introduced into the RS economy. Although in a VERY HIGH number, once they were discontinued, the price decreased to about half of what it was when they were unobtainable in the game.
