It depends on the context - your question is very vague, but in general distinguishing between cells based on what vectors they have taken up (and are expressing) is highly important. One way to do so is to add GFP (https://altogen.com/product/gfp-expressing-plasmid-dna-25-ug/) to your transfection, and then monitor for it afterwards. The step is important as helps distinguish between different cells/tissues depending on what kind of activity they are exhibiting.

I think in many cases it's very important to be able to distinguish between cells that have taken up the vector and those that have not.

Distinguishing transformed cells from non-transformed ones is highly important to the accuracy and possibly yield/ signal-to-noise ratio of your work. It can often be done by either monitoring fluorescence of a control GFP-vector, or e.g. antibiotic resistance of the transformants.