Splines in Revit work differently than splines in other applications like Autocad or 3ds max. There are 3 major differences:
1. splines in Revit cannot be trimmed
2. splines in Revit cannot be extended onces it has been created. Dragging the end points will only scale the created shape.
3. you can't make end points connect to create a closed loop. This is required if you want to create extrusions, blends etc. Creating a series of arcs to approximate a fluid spline in an extrusion or blend will result into unwanted vertex connection.
The workaround for number one and two is to export the spline to Autocad or 3ds max. Trim or extend it there and re-import it back into Revit. Once in Revit, you need to explode the spline. Revit will convert it into a Revit splines.
To create a smooth surface extrusion, blend or sweep from splines, start off in Autocad. Create the spline and make sure it is a closed loop (use the "c" shortcut). Import it into a family or inplace family sketch, Revit will convert the closed polyspline into 2 splines. If necessary, you can modify these splines, just make sure you don't move the endpoints. Once you finish the sketch, Revit will create a smooth surface extrusion, blend or spline with no vertex connection.