In THEORY using a 2.4 GHz digital control link as the Inspire does (and nearly all other modern model radio systems) should make it fairly immune to outside interference. Because it is a digital 'packet' transmission system, as each packet is transmitted it will have a unique identifier code attached to it. This means that the receiver will ignore anything that doesn't have the identifier and only respond to the unique code coming from the transmitter......IN THEORY.
Unfortunately, rf propagation can be a bit of a funny animal and as (in our case) the Inspire gets further away from the pilot the received signal will get weaker and weaker....kind of like someone talking to you and then walking backwards into a crowd of people all talking. You will still hear what they are saying but their voice will get weaker/quieter and start to disappear into the general 'chatter'. Now imagine you are just on the threshold of being able to hear them clearly and then someone comes right up to you and starts talking loudly in your ear. Your hearing would be swamped by the person yelling in your ear and you would lose the flow of conversation from your distant friend in the crowd.
Same thing with rf. A receiver can get swamped out by localised rf transmissions (especially on the same frequency or harmonic of that frequency) causing loss of signal at ranges far less than expected.
With the Inspire, if you have WiFi turned on you have an rf transmitter (your tablet) on 2.4ghz very close (inches) away from a 2.4ghz transmitter (and receiver) on your Inspire. At this very close proximity harmonics can play a role as well causing the front end of receivers to shut down and desensitising due to a strong localised signal.
Although manufactures work very hard to make their equipment as robust and immune as possible to extraneous rf, nothing is totally immune and there is always a risk of loss of control link caused by this swamping phenomenon.
In 999 flights, you may be OK but in the right conditions on the 1,000 flight it is possible you loose control of your craft due to this. Personally, I would rather not take the risk. I am of the opinion that anything I can do to minimise any risk (however small) can only be a good thing. I do not need WiFi or cell phone connection etc when flying so I take that 'risk' out of the equation by switching to airplane mode.
Sorry for long post, but hope that helps.