Ok, seeing as I do sell these I will do my best to give you an unbiased opinion.
While definately a "nice to have" it is by no means required.
The cell allows you to mount your 4 probes into it, however you don't want pressure to build up inside the cel, as this could damage the probes, so running it on your return is a bad idea (back pressure from head height).
The Cel is ideally suited to low pressure applications, such as before or after a UV, or T'd off from your return line.
Its a handy piece of kit that will always keep your probes wet and secure (very handy, trust me), but if you can't plumb it in the way it was designed to operate, it might be overkill.
Just my 2 cents.
