You will not be able to use that unit to finalize your mix--end of story. You need a stereo unit to do that. So in that respect, yes it is worthless.
However, it is not worthless as a studio device for other things like equalizing voice, or instruments like guitar or bass. It could be very useful for that.
If you are buying an eq *primarily* for the purpose of equalizing full mixes, then a mono unit is just plain the wrong tool and a waste of money. But again, that's not to say that it won't have other uses.
Also, $250 is not such a great price for that unit. The stereo version goes pretty regularily on eBay for around $350. One recently sold for $71!
You can easily find an Orban 622B (a stereo eq) in the $250-300 range.
The point being, if you need a stereo eq don't get a mono eq.