Here's what Vinnie told me about why the SMART circuit is still relevant when we can leave Gary's charger plugged in...
Yes, you can leave the charger connected to the SLA batteries - but the problem is that the charger gets "tricked" into thinking that the batteries still need to be charged in the "bulk charge state", which is 14.75V per battery, while they might already be fully charged. So this will premature age the battery. The reason is that the charger sees the load of the amp, and is tricked into thinking the batteries are not fully charged when they are. Leaving them charging in the "bulk charge" state of 14.75V per battery will dry them out. If a trickle charger was used instead of a two stage charger (with no greater than 13.8V per battery, this should not hurt the batteries).
Also, without a SMART board, when the batteries go bad (when running off the grid with no charger connected), the voltage drops and drops without the customer knowing, so you lose power and performance and the customer never knows it - until finally the voltage gets so low that the amp does not run correctly and shuts off. So it is good that the customer knows when the voltage is getting too low and does not play below a minimum voltage, as performance is lost.
Also, if you run with the charger connected at the same time, it is not off the grid and the charger noise is coupled onto the battery power (you can see the HF noise with a scope). In this mode, the batteries are acting like huge storage capacitors and they do clean up the sound, but it is not as clean as running w/o the charger, and like I mention, the batteries get beat up.