I had a similar conversation with our builder recently - which I think I understood - so allow me to summarise, and please correct me if I'm wrong!
We had wanted TRVs on the Rads and a thermostatic control on the boiler (to replace an existing timer control). Plumber put TRVs on rads, but was reluctant to put thermostatic sensor on the boiler because the house, although small, was not zoned. AFAIK, a new build of our size would have three zones - upstairs, downstairs and water - so that there could be a separate thermostatic sensor for each zone. We, on the other hand, have just one zone - which means for example that the thermostat would keep the boiler off on a hot day, and we'd have no hot water... Or if the downstairs were colder than upstairs, the heaters upstairs would be unnecessarily warm. So the compromise is a timer on the boiler, and we just use the TRVs for the individual rads... Which is not quite as efficient, but means that we won't keep on forgetting to turn off the immersion when we need hot water in summer!
That's what we did, and that's what I think your plumber is advising you to do - and it sort of makes sense to me - although I'm sure that Heinbloed is going to advise otherwise!