I am not familiar with how SolidWorks Simulation handles contacts, but the software I am more familiar with allows you to have either frictionless or frictional contact. Frictionless contact solves more quickly than frictional contact and higher coefficients of friction will usually increase the solution time more than lower coefficients. (Although you can also run into cases where frictionless will allow rigid body motion that will kill the solution.)
The error you will get will depend upon the details of your assembly. I once designed a spring contact to make electrical connection between two PCBs in a calculator. I did my analysis without friction and had prototype parts made to check it out. (We didn't have FEA at the time.) It didn't work. I went back and added friction in the analysis and discovered that my contact force dropped from 100 grams to 0. I had wasted $20,000 and over a month of time. So my advice would be to run the analysis first without friction and then run it again with appropriate values.
Can you run an analysis without friction? Yep. Just set it to 0. I do agree, it makes it look like you NEED it, but you don't.
Which takes less time to solve, no friction or less friction? Depends on the problem. Frictionless will take less time if the parts aren't interacting a lot. But, some problems need friction to stabilize the problem so without it, it might take forever to run.
Do you need it? It depends on the problem. Like Jerry said, in the real world it exists, if you are looking for absolute answers you need it. If you are looking for trends or the assumption to not include it is ok, then you don't need to have it. This comes from experience and comparison with a known solution.