Excellent points Bingo and ALL
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If the skill system is set-up well, then hopefully the additional work of a decay system might not be needed? (I try to think along the lines of; least amount of work for programmers, makes most in-game Fun, makes sense in-game, trouble shoot/safe-guards needed -to keep game balance & game speed at good 'levels.' <---And, open minded to any changes with that
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I could see skill decay being used in some ways;
1. If we had a system that 'abused' players being able to collect a 1,000 different skills & all of them would stay at 'high levels.' Decay would be a way of 'curbing' that abuse? But, a good skill system will hopefully avoid 'unrealistic' skills levels?
2. Decay of skills could be used as a curse, a disease, a protection / ward / glyph, a magical attack maybe? Gypsy casts curse, the warrior begins to slowly lose skill points in ... 'two handed swords,' etc. Inside the demon's lair, no healing skills will work. Decay being more of an attack in that sense.