Well, armours (normal and spawn ones) have many secondary effects: with the current system, it's a great pain in the ass to be forced to go to Ammu over and over again every single time you spawn, and this affects more the weak players who die often than the skilled regulars who survive easily, which helps to break the balance and equality. However, a spawn armour at a high cost would increase the problem of rich regulars rapping the poor newbies, creating an even more unbalanced game. And if the spawn armour were cheap and easy to get, everyone would use it, resulting in a more equal and balanced game but I think that having all players armored wouldn't be good either...
If it were up to me, armour wouldn't be buyable (neither normal one nor spawn one), and spawn armour would be automatically granted as a handicap for weak players who die often and almost never kill.