But.... but... you're only talking from your point of view... now play as a nasus and let's review these points!
1.If the team win (for the moment) let's go stack a bit more, because the enemy team might have a comeback. Not the only point, if my team can win a 4v5, then I can go to my "heaven" stacking the hell out of the creeps... win-win situation.
2.If we lose, then whatever champion you play, you should farm more and wait for some items, for sure later you have more chances of win. Same goes with nasus, even more I can say because of the stacking :D
3.Depends where the jungler is, but not all Nasus players don't help. If i see the "invade" in time, for sure i go help the jungler! He's one of the 4 buds i just met.
4.Finally, this point! Trust me, you CAN win a 4v5 if team si tower-hugging and dodging any harass abilities. Wards make it less possible for enemy team to get you by behind or unprepared (talking about 4v5 on mid), you just def. And when THAT nasus, who have been stacked like hell untill minute 30 and he has 600-700 damn stacks, says "Ok guys! I'm ready, all mid and end!", then you go with him 5v5, one last push and win!
5.Of course i get mad at anyone taking my creeps! I don't care about 150 gold, if my team knows to farm 150 gold is around 7-8 creeps wth ? I want to stack even more!
6.That is not true, "will always", you can't say all nasus players do that just because you've met many bad ones... And even if he does so, then he can clean up and push and maybe even take some objectives.
7.As I build full tank on nasus I think I should initiate the fights, and once a nasus is stacked up (with or w/o full build) there is no disadvantageous team fight ! :D
8.Actually, sometimes you can 1v5 with enough Q stacks and full build! :) But depends on the situation... Anyway it's better to go as a team, and not to risk.
9.Actually, if they get fed(meaning stacked up, here) and I'm not referring about 500 stacks in min 30 or 800 in min 50... If they really get stacked up in a good amount of time, then the game really is won :)
Now... all these points are from my perspective as a decent Nasus player... The lesson here is: If you want to make something, do it good! Not just from one point of view in this case :D
Later!