Okay, what's the real scoop on the need for
this when connecting condenser XLR mics
to 1/8" miniplugs that have DC power
running to them, such as some of the
Sony cams and minidisc recorders.
Supposedly there could be some type of
problem using an XLR-to-miniplug cable
if the cable does not have the voltage
blocking capacitor. I think there is suppose
to be a hum in the audio. Is there any
downside to using one of the cables
with the blocking capacitor if it is not
really needed?
Has anyone had any experience with this
topic?

There can be hum or worse, severe static crackles when using any mic into a powered input when the mic isn't designed to use that voltage. This mainly happens when there is any movement in the mini connection, which is really impossible to avoid.
I've never had any problems with using a blocking unit into an input that doesn't have power. I think any effect would be very slight under real world conditions.

Oh, that's a new one to me: never heard about the
static crackles.
I thought maybe it was just a gimmick to get
you to buy something you don't really need.

I can't comment on audio quality degredation but would like to comment from an engineering point of view.

The following is assuming you are using a battery powered condensor mic. The worst case would be that without a DC blocking cap you could feed the DC voltage from the mic to the mini jack on the camera and cause damage to camera's mic input circuitry. It is possible that the camera has DC blocking caps to protect from this. The reverse is also possible but my SWAG is that is less likely.

Unless you have accurate schematics of the mic itself and the cameras mic input it is impossible to know exactly what is being connected to what.

For me I would rather be safe than sorry. If it is recommended that you use a cable with DC blocking cap(s) I would do that.

The cap's value should be such that it has no effect on signals in the audio frequency range.

Don't ask me about the time I forgot the $30 DC blocking cap on the input to a $30,000 piece of test equipment.

Don't quote me, but I thought I had read on the
site of BeachTek or Studio 1 that their
bolt-on boxes have the capacitor.

But that's good to know that the cap should
not degrade the audio because I recall
(but not sure) that someone said you shouldn't use
the cap if you don't need it; maybe the reasoning
was just so as not to waste money.

I have both cables (cap and non-cap)
and have thought to test it out but never
really gotten around to it.