koninklijke
Picked up while ago -when leafing through the wik-i that in unifying areas you'll also be unifying their version histories and tags - history is, and tags may be, different, which may stand in the way ... . . 
Just checked wiki.OSM, found in only in  Merkaartor documentation : "Join Areas :
 Join two selected areas that share a common way. The Tags from the two areas will be merged in the new joined area." - no mention of their version history being retained or merged, or the internal boundary being removed ; taken steps above in iD, with request that changes be reviewed. . 