Shared control really depends on the situation. First, a few things to remember:
- Shared control does not have to be permanent for the game
- Shared control does not have to be mutual
A few scenarios where sharing control in a random team would be beneficial would be;
- Early rushes, such as combined or mixed zealot, zergling or marine rush. There is a huge benefit in making these combined rush armies act as a single army instead of scattered control. This also allows the off player to focus more on economy and tech instead of micromanagement.
- Terran walls, and only during the early game.
However, shared control is much more beneficial when you have a trusted partner that you play with often and have a good line of communication with. Shared control in these scenarios allow you to act as a redundant set of eyes for each others armies. For example, units or armies that stray out of position can be saved by a command instead of mad pinging and yelling.
Shared control with an ally also allows better use of spell casters or micro management of certain blocks of units (e.g. one guy harass with all available air while the other distracts with all available ground at the front door). There are many different scenarios which this can be useful, not restricting strategy to the owner of the unit but instead the type of unit.
If there is no line of communication (usually in random teams), then sharing control is usually a bad idea. You can not predict that your strategy will align sufficiently with theirs to prevent unit control conflicts.