I just got out of a game with a jungle Heimerdinger, Top Irelia, Bot Nunu+Kogmaw, and myself Mid as Veigar. It was a loss, but I believe it was only so because Nunu and Kogmaw were pushing bot when the entire enemy team zerged through mid inner tower, inhib tower, inhib, and nexus towers.
Anyway, Kog and Nunu blame me for not ganking bot (Kog ended with 7 deaths). They say "The enemy team had jungle and mid presence in bot lane and we didn't", even though the enemy mid (Vladimir) never ganked bot either and the enemy jungle (WW) only ganked once. They say that they would have won their lane if I ganked.
However, I know Veigar is like Nasus in that he needs to farm his Q as much as possible in lane. Even without that, Vladimir was pushing mid like a madman and would certainly have gotten the tower down if I left to gank bot. Even without THAT, I was told by the enemy bot lane (Ashe+Galio) that they had constant wards up.
I ended up getting 100 on my Q by 25 minutes, and having 2 doran rings, sorc boots, Deathcap, Deathfire grasp, and Fiendish Codex. My scores were 4/3/2.
So, tell me: Is it a given duty of mid lane to gank the other lanes, even if you are a farming mid like Veigar?
Enemy team was: Top Udyr, Mid Vladimir, Jungle Warwick, Bot Ashe and Galio