Hi forty, yes for more usual construction it tends to be good to have as simple of a base surface as possible and then any irregular boundaries are trim curves. Constructing things for Flow in particular can be a little different since you sometimes may want a more complex surface since the low level surface structure has a big effect.

Another reason why Construct > Planar builds a trimmed surface is so it can support any kind of complex irregular boundaries like this for example:



It would be extremely difficult to create a single surface that tried to directly follow all the contours of that outline, meanwhile the method of constructing a large base plane that has trims on it is able to do it.

- Michael