Yes, restrictions on mobile phones on the apron are possibly excessively cautious. However, they are higher power than radios or even short range network phones, and might produce arcing.
There are some really unexpected EMI effects - even within aircraft systems. To the extent that an apparently insignificant area of damage on a piece of cable in (e.g.) the transponder system can cause a significant failure in a totally unrelated system such as pressurisation outflow valves. The point is that aircraft are large and complex with lots of electrical systems interacting in very subtle ways. Even establishing that EMI for particular frequencies is not a problem on an aircraft off the factory floor doesn't guarantee that it won't be for other aircraft off the same line, or the same aircraft when somebody has made a nick in a piece of cable somewhere.
I'd go with hardening the aircraft to military EMI standards. Then we'd not be able to put so many passengers on, because of the increase in weight, and nobody would want to fly because tickets would cost 30% more. That would improve turnaround times too.
Seriously though, outofsynch, I'm not too bothered about potential pax grumbling about excessive safety restrictions. If you really have a problem about being told not to use your mobile phone on board or on the apron, then go by car. That's one less whiny passenger that my crew will have to deal with.