My main reason to float my 'scope is because of bridge rectifiers combined with
grounded transformer power supplies.  With HVAC systems, one side of the 24VAC
transformer is connected to earth ground, which means that a grounded 'scope
will cause problems connected to the output of the bridge rectifier.

Also, when using the in-circuit emulator, combined with the bridge rectifier on
my target system, I must float the target system, as my computer is grounded
through the network cable (ethernet) and it isn't practical to isolate it.  At
that point, I usually float the 'scope, so I have my choice of using the
microcontroller ground (Vss) or chassis/earth ground as a reference point.

One problem I have had is that my usual 'scope RF line filter can allow enough
current to flow to prevent my target system (with windowed/OTP PIC) from
resetting properly when power is removed.  When that happens, I must remove the
oscilloscope ground lead from the target system to get a good reset.