Lots of good reasons to use the paired shaft rocker arms. For starters, they usually have a lot more options in terms of rocker arm ratios and offsets. Also, they use smaller shafts which allow them to have a smaller body which allows more room for big springs.
The quality of the parts is usually quite a bit higher and there are various options for the style of adjuster.
The rocker arm geometry is usually better since the stand height can be adjusted with flat shims rather than dorking around with a cast stand. The paired shaft systems are more rigid since they aren't whipping around on the cast stands.
When you assemble and disassemble the engine you rotate each cylinder to TDC and then pull the rocker shaft for that cylinder. That way you aren't pulling the threads like you have to do with a common shaft system.
Really, if you're going to get serious about making lots of power or spinning to a high RPM then there isn't even a debate about what rocker arm system to use.
If it is a show poodle then you don't need any good parts at all, just as long as it will start and drive to where you want to show it 
The perforamnce advantage of using any good racing part is it does not fail in competion until it reaches it usable life span, which all race parts have. The cheap parts can kill you 
