Veterancy adds greater depth and makes games more interesting. There have been plenty of entertaining moments both in games and watching casts where for example an ACU is about to die and gets a sudden rank of veterancy allowing it to just escape a kill. It also gives a spotlight to units that have a massive effect for their cost (e.g. units that reach 5 stars).
However Ive no issue with allowing a unit's mass kills and veterancy progress being visible to all players (if it was possible)