That is wastage...pure wastage...DONT EVER DO IT..

plat armours are only BEST USED WHEN COMBINED WITH 3 FAIRIES...
That gives u 8k exp

...if u combine 1 plat armour and 1 gold unit u may get 10k exp(say)
this gives only 2k extra exp. only

bt if you combine plat armor with 3 fairies..then combine gold unit with 3 fairies it will probably give 20k exp..

these no. are not accurate..only approx.

In short dont do it..
use only plat armours and fodder units...dont waste ur gold units just for lv up..save them for awakening...