the problem is that with the testing ban senna would have 1 session to not only get used to an F1 car but also to decide what areas he felt should be developed. then he would be visiting a fair few new circuits in a car he wasn't that familiar with at a level he was not familiar with. barrichello has a proven development record especially with brawn and can bring a car home which is what the honda team desperatly need this year ( what are they calling themselves now? i assume it's not honda as it the "honda powered by mercedes" bit would be very embarrasing )
button has a reputation (true or not) for letting his head drop if the car's not competative and last year barrichello outscored him (yes the podium was lucky but so was buttons only F1 win and they both had to get the car in the position to benefit ). also if senna was with him, he would have no one to judge him by apart from a driver with very little time in the car and i suspect his team need to measure button's desire before next season. if he fails to beat barrichello this year then questions will be asked.
finally of course, if senna with his lack of experience damaged a few cars heavily it would offset some of his sponsorship money and the team would be labled as one who chose drivers on size of their (sponsors) wallet rather than ability plus the knock on effect of spending time and resources repairing rather than developing.