Old sheetlets cancelled on recent days

For some collectors, they don't wish the old stamps used in modern days' mail due to their logic is it looks oddly and unreasonable. However, once the stamps still have their value, and post offices accepted. Why do they ignore them outside their collection? Here is an example of Solomon Islands.

I bought two souvenir sheets in the stamp fair on February, as I missing them and they are selling in bargain price. So I try to affixed them on envelope and card then send to Solomon Islands Philatelic Bureau. As my wish, they done and return to me after a while. The first sheet issued on 1982 to commemorate the stamp exhibition in Australia, features four common birds to be found in Solomon Islands and Australia.

From left to right :
12c : Little Pied Cormorant (小斑鸕鷀) ; 18c : Pacific Black Duck (加姆島野鴨)
35c : Nankeen Heron (棕夜鷺) ; $1 : Oriental Dollarbird (三寶鳥)
Solomon Islands (1984)
2nd April, 2008. Honiara

Second sheet was issued on 1996. I owned block of 4 of whole set stamps for a long time but missing the sheet. Sadly, the sheet colour looks fade out and no attractive. I decided tear the stamp out and becomes below maximum card.

Massena's Lorikeet (馬仙納氏吸蜜鸚鵡)
Solomon Islands (1996)
2nd April, 2008. Honiara