The Doubler skill has several benefits, which are all very important in order to become a high level player. The first is the increased stamina burn rate which allows a player to burn more stamina each hunt without having to re-buff. It also has the side effect that hunting is quicker, giving players who cannot play FS as a full time job a chance. The second effect is that it increases the kill-to-move-ratio which essentially means that a player with Doubler activated will spend a larger percentage of his stamina on creature killing than on moving. On areas where the number of creatures is scarce, this benefit is even more obvious.
Furthermore, the Doubler skill also helps against creatures which has a high probability of killing the player, making it easier for the player to end the hunt with a positive XP and Gold gain. The reason for this is that doubler only affects the gains when the player wins a fight. When the player is killed, Doubler doesn't apply and the Stamina, XP and Gold losses are at their normal levels.
Kill-to-move Ratio Explained
Example, suppose you oneshot all your opponents, you find 4 mobs on each square (average), you always spend only 1 stamina to move and you want to burn 800 of your stamina. Each square will cost you an average of 4 stamina to fight and 1 stamina to move. So, of the 800 stamina spent, 160 will be spent on traveling and 640 on fighting. Now, do the same with a doubler *3: each square will cost you an average of 12 stamina to fight and 1 stamina to move. So of the 800 stamina spent, about 738 will be spent on fighting and only 62 on traveling. That's a 15.3% increase in both XP gain and gold gain.
Here is a recapitulation of the increased XP and gold depending on the number of mobs found per square on average (this depends on how many players are roaming the same map you are)
Assuming you can one hit the monsters and you miss only 2% of your strikes:
- 2mobs/square: +28.1% (average XP and gold / stamina including travels)
- 4mobs/square: +15.1%
- 8mobs/square: +7.8%
- 2mobs/square: +32.7% (average XP and gold / stamina including travels)
- 4mobs/square: +17.3%
- 8mobs/square: +8.9%
- 2mobs/square: +42.0% (average XP and gold / stamina including travels)
- 4mobs/square: +21.5%
- 8mobs/square: +10.9%
Assuming you need 2 hits to kill the monsters and you miss only 2% of your strikes:
- 2mobs/square: +15.1% (average XP and gold / stamina including travels)
- 4mobs/square: +7.8%
- 8mobs/square: +4.0%
- 2mobs/square: +17.3% (average XP and gold / stamina including travels)
- 4mobs/square: +8.9%
- 8mobs/square: +4.5%
- 2mobs/square: +21.5% (average XP and gold / stamina including travels)
- 4mobs/square: +10.9%
- 8mobs/square: +5.5%
The negative effect is that Doubler also increases the minimum stamina necessary to attack. A 20 stamina attack normally requires the player to have at least 20 stamina remaining, but this limit is linearly increased with the doubler rate. So with 4x doubler activated, the player needs to have 80 stamina remaining to be able to attack. For 10x doubler, the player needs to have at least 200 stamina in order to attack. This can of course be worked around by removing doubler when the stamina is low, and then hunt the remaining stamina without doubler or with a lower level doubler activated.
When hunting for items, or when doing a quest that requires a certain amount of kills it is not always desired to have doubler activated. The reason is of course that the player's stamina will drain faster, possibly preventing the player from finding the item or completing the quest.
The gain will be somewhat higher if you have conserve activated. Note also that if you don't have doubler activated, you will more quickly deplete the zone of its monsters and you'll then have to wait or suffer a low number of monsters per square. As a side note, Doubler potions can sell for a very high price on auction, so you are probably better off buying your Doubler*3 or Doubler*4 directly from a player for somewhere between 5 and 10k gold. Doubler also saves you money on repairs: 4 times less fights means 4 times less repairs to pay for...