many specialty rulers---not just fons & porters---are 'helpful'
they are not necessarily required- you can 'make due' with what you have-
but they make things a little easier-
so if you have problems with certain blocks/cuts sometimes purchasing the special ruler will help you be more successful-
sometimes if you tend to do a certain block ALOT---like making many many flying geese blocks- it makes sense to buy a flying geese ruler-
they help with accuracy & with trimming- but you can still make (most) things without them- it is a personal choice
i have some specialty rulers-some i use alot- others never- it just depends on what type of quilts you make- and what is easy for you-
often your lqs will demo-let you try specialty rulers- so you can see for yourself the benifits and decide if you would like to buy it or not (and yes-they even carry the fon's & porter rulers---at least some do)