You can use video to simply get to know the other agencies - their upload systems, forum support etc...
If you are starting out new with photos, your niches have already been taken by someone else and you will have to keep at it for at least 18 months to get to similar levels.
But if you love the freedom of working with many different agencies and also working with a few that take exclusive images, I dont think you will regret it.
But if you have no time and cant face starting at the bottom of the ladder again...then maybe just branching out with video might be enough to add more balance.
I love being free and would never go back to full artist exclusivity. But it is not for everyone.