If you want to use a financial argument to justify the purchase of a sickle bar mower for $7000 so you can recoup the grass cuttings, I hope you have a lot of roadside to mow!
OK, roughly 43264 square feet per acre, and roadside should yield about 1 ton per acre, unless you are cultivating / fertilizing the roadside also. Let's use the 1 ton /acre, you can adjust if you think it's warranted.
An 8 foot swath of roadside would need to extend 5408 feet to equal an acre, close to 1 mile. So, 1 ton of grass / hay per mile of roadside. $100 value to the hay, $7000 for the mower, plus fuel, wear & tear on the Toolcat, repairs, etc. You need to mow 700 miles of roadside to cover the mower attachment cost alone.
Now, if you have to mow it anyway, and need to buy SOMETHING to mow it, some of that cost is sunk (you gotta do something) so if the incremental cost of buying this particular mower attachment isn't too much more than the cost of the minimal mower you need to do the same job, maybe it makes sense.
Personally, if I had the money I'd buy it just because it's cool! I would love to chase down a neighbor with that sickle bar running at full speed next time I catch him throwing rocks at the horses...
Rich