Optimizing a wide-band PA is pretty difficult.( Efficiency,Power Output,Gain,Linearity)
But a narrow-band PA can be optimized easier than wide-band.
And also, a Transmitter-if there is- will generally work at single frequency at a time even it's capable of transmitting multiple frequency.So, it doesn't make sense to occupy whole spectrum for a single frequency.It will also be eventually power consuming system and low efficient.
So, I would prefer a narrow-band PA instead of wide-band one due to those reasons and probably other causes.