Coins and Stamps in your Investment Plan?

Is it a good idea to put coins and stamps(and some Art) in your investment plan?
I think it is a good idea to put coins and stamps in your long term plan of investment, like stocks and bonds. The prices in the last 20 years (since I've started to collect stamps and coins) rose by thousand percents. Now days there are less and less stamps due to the postal services and technology (e-mail) and the governments that issue coins due to metal prices that rose also thousand percents in the last 20 years.
I can tell you that my catalogue value of my first stamps album rose from $1000 (20 years ago) to $6,000 those days, so if it will keep that way(I think it will) I can sell that this stamp album for $8,000 in 5 years, and give the other albums to my kids so they will keep and grow the stamps collection and also the coins collection.