Wrong, its only terrible if you aren't building items to help your team out the most. Any person who plays just for themselves is bound to have a losing match end game. Seen it happen it many times and nearly always happens. Not saying your reasoning for why Ashe needs an IE is wrong, but I don't agree with your rationalizing that every build besides what you recommend is 'terrible.'
Builds sometimes need to be altered for certain situations, but even then it still comes down to min/maxing. You may be doing fine with your bloodthirster on Ashe, but you won't be doing as good as you can, which is terrible. Not every build I recommend is entirely min/maxed, I realize that. Just like everyone it is what has worked best for me. I just go to lengths to figure out what items will suit the character the best. Not every build I recommend is the absolute best never fail build, I'm not the worlds foremost expert on every champion. I'm just saying that the build recommended here doesn't maximize Caitlyn's effectiveness as much as my preferred build.
I don't just play for myself, I will build my banshee's veil sooner against a heavy mage team. I will build a frozen mallet on a carry if there is someone else who needs the red buff more and I can't get access to the enemy red.
In this situation where it is bloodthirster vs infinity edge on Ashe:
- Ashe has a passive that frontloads her damage to guarantee a crit if she hasn't attacked for a while.
- Infinity Edge offers the most damage of any carry item, Ashe has a long range and slow so she doesn't need to rush lifesteal.
- Most of Ashe's damage comes from autoattacks, which means a crit build will innately be more effective than others.
Since she will be doing more damage she will be helping her team more, its that simple. She is a carry and should build for damage. It isn't selfish, by building more damage items she will be helping her team more. Playing selfishly would be a Rammus refusing to get magic resist because of his passive. I don't see how your selfish playing point fits in to what I was saying.
Terrible may be a strong word to use but it is at the very least bad if you aren't maximizing your effectiveness in your role to the team. I shouldn't have to say that builds should sometimes be altered depending on the enemy team and your team and what you should do if they have less armor etc. It is just a guideline to the most effective build in a standard match with standard team compositions. You shouldn't be building last whisper if your opponents aren't building much armor, black cleaver would be better in that case. But I just assume people will know this. A guide shouldn't be followed like the word of god, it is just that: a guide.