Main reason to leaving it outisde is for the indoor air quality.
A porty's exhuast blows out alot of crap. So its best be outside, but not always practial. i.e. not long enough hose length/machine doesn't preform well with such a long hose run, weather condtions, chavs hanging about outside, if your doing upholstery its not practical to go outside to turn off the machine constantly etc
Its rare that a waste tank or solution tank would start leaking on you but yes, the machine can leak from other places i.e. the pump and/or brass connectors inside the machine. Usually just a wee drip tho so no biggy.
Always best to put a thick towel or dust sheet under the machine and creating as much ventaltion as possible (open windea and doors with customer permistion) if using the machine indoors.
Tony