Like so many of these things, it's probabilities.
Scenario #1 You're trundling about in your Cessna. You're not using VOR, DME, SSR, ILS, just comms. If the GPS does put out a spur, it's under your control.
Scenario#2. You're up the front end of an airliner, on a dirty night. You're needing comms, ILS, VOR, DME, outer and inner marker beacons, SSR and TCAS at various stages. Now, can you guarantee that a spurious emission from a GPS in the hand of someone down the back isn't on one of those frequencies?
WiFi equipment is Type Approved - in Europe, it has to meet ETS300 - 328, and even there, the spurious emission level of -36dBm below 1GHz and -30dBm above is a bit on the high side if the spurious output is in the wrong place. The FCC limits are somewhat tighter, being -41.25dBm above 2 GHz. Most manufacturers, however, design to the US limits.
But it's all a matter of probability. Wouldn't be nice, though if the GPS radiated a signal on 1030MHz!