I’ll admit that I have a love/hate relationship with EVE Online. The space-based, PvP-centric game has its high and low points, but one of the things which they have done right is in their skill system. What they did was ensure a way for players to constantly progress with astoundingly little possible harm to their skills through PvP.
The basis of EVE’s skill system is training skills over real time. You accrue 1 SP per second, and skills are ranked from 1 to 5. Each level of skill takes a certain amount of Skill Points, and then you gain a level. Levels enable bonuses to your damage, the ability to use new abilities, equipment, or ships, and also unlock higher abilities for you to skill up as well. The later skills have base multipliers which affect the time needed to train: a 1x skill may take 4 days to go from 4-5, but a 4x may take close to a month.
Most people would say the main reason why they choose real-time over activity based is to keep you playing longer, or to keep things balanced between new and old players. This is true, but I think they did so also because of a critical need for PvP games: a measure of safe, uninterruptible progress. When your ship dies in EVE, you eject into a agile, hard to catch lifepod. Only when your pod is destroyed does your player get killed. You then awaken in a clone which should hold all of your SP if you set it up correctly. Your loss is your implants, and while it may cost a great deal of money, the result is to slow your time only. Only if you fail to set a clone at the correct amount of SP your character has are your skills harmed, with 10% taken off as a loss.
Even after death, you can still progress. Your training to learn new skills cannot be halted, even if you are in the losing end of a war. That means over time you can get stronger with no real risk to yourself. A level or activity based skill system would be vulnerable to other players ganking you and disrupting your party, or be slowed or forced into not fun gameplay in order to dodge assailants, like Darkfall Online’s macroing.
I think the general principle is to enable safe advancement in player power, while risking monetary advancement and gear power. This lets even losing players feel engaged and constantly improving in the face of loss. It’s a good idea, and one many traditional PvP games should take. Section off one aspect of advancement and allow it to proceed unhindered, but have other advancement trees where risk is always present.