Hello, and welcome to this guide which may or may not become irrelevant next patch.
I'm a lowly and slowly climbing EUW Silver player, playing mostly Swain, Senna, Pantheon and Yorick.
This guide is going to hang on for a while, before I manage to find time for a proper formating.

But now - why pick support item Senna at Top?

Pros:
+ Increased Mist drop rate from minions that you don't kill;
+ Aggressive nature of Support items in terms of gold gain cooperates with Senna's passive;
+ Increased passive gold gain;
+ Increased early mana regen;
+ You get to share CS gold with your Jungler;
+ You get more vision, which Senna severely needs to avoid being ganked;
+ You have good enough CC to support your jungler's ganks;
+ Your opponents are likely to underestimate you for your low CS.

Cons:
- Some match-ups are impossible to overcome;
- You get much less gold against turtling opponents;
- You are vulnurable to ganks;
- Your teammates are likely to flame you for your CS;
- You are playing Ranged top, so... flame incomming.

Overall, building and abusing your Support item alows you to gather far more Mist, thus gaining more free stats earlier than regular Senna builds and playstyles. Going to continue writing this Guide later on... if it is any good.