Longer timing belt life. The timing belt should "wander" less with the extra guide plate. It can only help. I can't see any down-side. Worst case is that I might decide to remove the extra guide plate if something unforeseen occurs, which I doubt will happen. But that will be easy to do, just remove the plastic cover on the right side.
Longer timing belt life, huh, the things look new after 20 years and 100k miles, good luck with that.
Wander less, what proof do you have that it wanders at all with the single guide?
potential downside: maybe the belt does wander a bit on the right pulley and without a guide plate its no harm, no foul. But with your clever modification the belt starts chafing on the guide plate and frays itself into self-destruction. Self-inflicted timing belt failure, engine destroyed, for nothing. Not likely that this would happen, but in my mind, why give it the one-in-a-million chance that it could happen? You're not fixing anything, so why even go there?
If a professional engine designer with intimate knowledge and testing of his design purposely left it off of that pulley, why would you second-guess that decision for no real benefit? That really was hard for me to type because I often challenge the Honda engineers on their decisions like the floppy saddlebag mounts, requiring crazy amounts of plastic removal to access the radiator cap, etc. But one thing I've never challenged is that the ST engine is totally bulletproof as-is, so why mess with anything about it?