If you have a grounding probe you should also have a GFCI - without the GFCI you will have current flowing through the tank, and that tends to piss off your inhabitants. If you just have stray voltage with no current behind it, it's not as much of an issue.
I don't have a grounding probe (too cheap and lazy, really) but my GFCI has saved my life and my house on a couple of occasions.
What the GFCI is doing is basically counting electrons - if 2 go in and only 1 comes out, we have a ground fault and it kills the circuit. You could basically throw a hair dryer into your bathtub and if it were on a GFCI you shouldn't even realize anything had happened. It's the ultimate failsafe.
The grounding probe just draws stray voltage and dumps it into ground. If the voltage is coming from a defective electrical appliance THAT IS BAD. If it's simply voltage induced through your lighting or whatever, then that's one way to funnel away the voltage. With a combination GFCI/probe, if an appliance is leaking voltage into the tank, it should trickle to ground via the probe - then the GFCI detects that and kills all power. Then it's your job to unplug everything, reset the GFCI, and plug everything back in one by one until you find the offender... and then throw it in the garbage Even a "little" current in the water is a bad thing and you don't want to be playing with that. Considering all the money, blood, sweat, and tears you've thrown into your tank, it seems silly to risk it all (and your home and your life) by letting a known "bad" appliance keep leaking.