Antennas and consequently also the BALUNs attached to them, are very frequently subjected to high voltage surges, caused by induction due to lightning or static charges.
The levels of these surge voltages are many times higher than the normal working RF voltages applied during transmission.
As a result, the insulation of low power BALUNs is continuously stressed, potentially resulting in microscopic burn holes in the dielectric insulation of the windings.
These micro burns often do not lead to immediate catastrophic failures, but cause a considerable lifetime shortening of the parts.
They degrade much faster, eventually causing a short circuit or arcing fault.
High power rated BALUNs use windings with much higher voltage withstand properties than of the shelf low power amateur products.
As a result, they can withstand considerably higher surge voltages, and do not suffer degradation over time.
So, for sure, it makes sense to use high power rated BALUNs, even if you’re running low power only.