It's part of the game. In DOTA style games, it's a much more obvious skill because everyone is limited to the same level of farming and the most efficient farmers are rewarded obviously and immediately. In an MMO or diablo type game, I agree it's rather brainless and simple but it is still part of the game, regardless. Depending on your strategy or methods you might farm much faster than another player. Or slower.
In DOTA games, it's obvious the effect farming has, when some super farmed carry easily destroys you or your whole team. Yet it does indeed require skill, otherwise everyone in every game would just "farm to win". It doesn't work like that. In other games where you don't directly observe the farming, it's less skill intensive. More time spent at farming gets you better gear. Sorta like more practice at PvP makes you better at it.
Practicing pvp to increase that skill is valid in your opinion, while spending more time farming better gear is not a valid strategy. Why?