When you queue up as an adc or support you become committed to the other player. This is the most important combo of your team. Supports need to build for what their adc is missing. For instance if ashe is your adc you don't need slow. A sheild or speed boost would help much more. Protect your adc instead of roaming or chasing kills.
And you should chase kills, ward, and roam when youre supposed to, not spend the whole game being a meat shield for ADC
Unless you are Bard. Then just build full move speed and roam all game.
I agree with what you're saying to an extent, but you shouldn't build to fill in the missing pieces, so to speak. You should build to enhance your team comp and possibly counter build in order to stay alive as long as possible to help your team as a support.
I think this is true in some ways.
Maybe for early game adc and support have a strong relationship but, I also agree with some of your comments. The support is there to support all of the team. As Aeolian said, it is better to build considering the need of all the team.
I will bring another thing tough. You should also build considering some members of your team is fed or not. For example, I will build a Locket if my team needs defense/magic resistance because of a fed enemy but I will buy a banner of command if we are fed.
P.S.: Sorry about mistakes in English.
