One of the things you need to remember is that most fixtures reduce the water flow in their internal ports....so even if yo uhad 1" pipe running to each fixture, you'd only still have the reduced volume on discharge. An exception is the high volume roman faucets used for whirlpool tubs...the use nearly a fully port. Measuring the pipe is one thing...measure the internal diameter of the fittings...it's closer to 3/8".
Another important consideration is the overall design, is it a home run design, modified with manifolds, or "typical". That will determine you flow needs and hopefully your plumber is taking that into consideration. FYI...I have have a bath group (2 vanities and shower) on the same run and the 1/2" line works well. The only problem is it's about 70 feet, so the first one to use hot in the AM has to wait a bit....but it's not too bad. When I built the house I used a home run design but each hot had the hot delay...so I moved them to one line and it's worked well.
I'm really happy with the PEX...no reason to not use it in residential construction as far as I'm concerned.