I have used a longer bar on my 7m and it works fine. I quite liked it.
But, I much prefer having a bar for each kite.
The lines wear out faster than the kite so it's a good idea to spread the wear over a number of bars.
Having another bar is great if you break something or leave a bar at home.
It's going to be much easier to sell a kite with a bar than just a stray kite by itself.
It should be much cheaper to get the bar with the kite than to try and buy a separate bar if you need another one later.
It usually makes sense to conform to the manufacturer's instructions. They might know a bit more about it than you do.