I can give a few anecdotal data points on this ...
At a previous job, the bosses had a nice house out in the sticks - overhead for both lecky and phone. They regularly lost equipment to lightning surges - but ONLY equipment connected to mains and phone (e.g. the fax machine). I don't recall any kit loslost that wasn't connected to both services. According to the regs, high risk and needs SPD - even though anyone who understands the subject will tell you it's as much use as a chocolate teapot if installed as per the regs.
Conversely, round here it's all underground services so naff all risk. The main risk is likely to be loss of PEN and over-voltage to sone users - for which an SPD is no help (it'll either blow it's fuses, or conflagrate itself).
For the general case, an SPD isn't likely to be much help unless you route all services via one point - and connect SPDs between each service and a co-located common earth point. Just a few feet of 10mm bonding cable might as well not be there when talking about kightning induced surges.