Why is this any worse than loading railway vehicles to take them to workshops because it's cheaper than the track access charges they'd otherwise incur?
Is that why they send railway vehicles by road? Or is it because there are no paths available for them to go by rail? Or are these things different sides of the same coin?
Both reasons would be valid and they both emphasise the same point - the railway is not always the best, cheapest or convenient way to get things from A to B, even if that those things happens to be railway railway vehicles of one kind or another. The same equally applies to railway staff.
It has certainly happened to me, although of course I haven't been on the railway payroll since 1980 so that particular element of the comparison doesn't apply. There were four occasions over 15 years when I took a domestic flight to attend meetings in client's premises, none of which took longer than three hours. There were two Bristol to Newcastle flights, one Bristol to Glasgow, and a Heathrow to Aberdeen. Flying allowed all three of these meetings to be easily-manageable day trips, something that would not have been realistically feasible by road for any of them, or by rail for the Aberdeen meeting, and would have made for an exceptionally long day (if possible at all bearing in mind the times of the last trains home) for Glasgow and Newcastle. And making any of them an overnighter would have put accommodation charges on the client's final invoice.
The article mentions Cardiff to Anglesey (which I presume to be Holyhead if railway staff are involved in needing to go to the island). Leaving the relative costs to one side,
RTT» tells me that that can be up to a 5-hour journey in each direction, so a member of staff would be away for at least 12 hours if they went by rail. Google tells me that Flybe operates two flight a day which makes a day visit possible, with a flying time of 50 minutes each way. By going up on the 0725 departure from Rhoose and back on the 1725 the staff could have a full day there and still go to bed at home that night.
It is perhaps important to remember, although going slightlly off topic, that it would have not been any better before the pre-Beeching closures. Whilst it would have been possible to the journey entirely within Wales, going via Carmarthen - Aberystwyth - Dovey Junction - Afon Wen - Bangor, it would hardly have been faster than using the Newport to Shrewsbury line.