Bassically it's about if you want HP being primarelly about class, or about CON. Who should have more hit points? A wizard with a very high CON, or a fighter without it?
If you add CON by level, then in the long run CON is much more important than class for hp. In 3.0, a wizard might have CON 14 and then get, eventually, a +6 item. That means a lvl 10 wizard will have 10d4 (average 27, including first level maximum) from class, and 50 hp from con alone. A lvl 10 fighter with CON 10 would have (if such thing exists and survives) 59-60 hp
If you add CON each level, CON becomes an uber-important stat for everybody. It's simply too rewarding to raise CON, and to dangerous not doing so. The difference between a CON 10 wizard and a CON 18 wizard (with stat boosters) is simply too high. Much more important than any other stat: WIS 10 or 18, or DEX, or STR or CHA won't give you such a big bonus.
If you add CON by level, then in the long run CON is much more important than class for hp. In 3.0, a wizard might have CON 14 and then get, eventually, a +6 item. That means a lvl 10 wizard will have 10d4 (average 27, including first level maximum) from class, and 50 hp from con alone. A lvl 10 fighter with CON 10 would have (if such thing exists and survives) 59-60 hp
If you add CON each level, CON becomes an uber-important stat for everybody. It's simply too rewarding to raise CON, and to dangerous not doing so. The difference between a CON 10 wizard and a CON 18 wizard (with stat boosters) is simply too high. Much more important than any other stat: WIS 10 or 18, or DEX, or STR or CHA won't give you such a big bonus.