I imagine doing that would give a lot of possibility to really livening things up at times. With all the drama coaches often bring to the sidelines in other sports it'll really be interesting to see how it is handled in Tennis.
I personnally don't like the idea.
Tennis can be a team effort (Davis cup, double) but it's about an individual performance. Then the most complete player wins (with own fighting spirit and tactical clue findings for instance...).
Getting an help from the bench is precisely in help for those who can't manage themselves. Don't like this idea...Of course a coach could help a player to level up her game to her opponent...But then, who will you give the cup ?
Therefore, my opinion is to keep coaching action in team tournaments such as the Davis cup only or out the match.