Hello. Fair enough that is good question. I will just point out a few things to consider for yourself to possibly help you make an informed decision on this matter.

1) I would not recommend you have them just spray the walls. Here is why. Firstly, I like to see the paint being worked into the surface with a roller rather than just sitting on top with it being sprayed on. This is especially true for primer. A level 5 finish will have so much drywall dust on it the primer really needs to be worked into the surface with backrolling after spraying or just by rolling on the primer. Why you may ask.......for proper adhesion is the most important reason.

2) Spraying without back rolling - I am not a big fan of this procedure. Here's why. Walls just sprayed will not touch up well. One will not be able to match the look of a sprayed surface with a roller. Its just not possible.

Spraying isn't really the most efficient way of doing a job always. It's great for emty unfinished homes with no flooring or furniture but just not effective for places with flooring or furniture and with the time spent covering things and masking off floors and ceilings paint could be going on the walls by brush and roller. In lived in spaces spraying is too messy having overspray floating around making things very dusty.

3) Finishes

While spraying looks great on trim and doors it is my opinion is doesn't look great on walls. It has more sheen (is more shiny) when sprayed with no rolling. The surface just sprayed will be less wipable and durable as mentioned above with the adhesion issues that can occur with spraying only.

Great finishes can also be made by doing things by hand (meaning a brush and roller rather than spray equipment). It just takes some knowledge and skill. Things like using a paint that is good quality, using a a high quality roller sleeves, and using good techniques in applying the paint and backrolling. One could also use a roller sleeve with less nap (nap is the length of the fibres on the roller sleeve. The shorter the nap the less stipple that should be left. An example would be that a 10mm roller sleeve with leave a tighter stipple than a 20mm sleeve). I like 12mm or 13mm sleeves for walls.

With these things in place and a good tradesman the final results and look should leave you very happy. I hope this helped. If you have any further questions let me know and I will give you my thoughts.