Yes and no.... or in other words... it depends...
Yes, adding a sub will be "ear candy" for the mix you hear in your studio. I too love the punch the sub brings to the game.
For reference.... I mix on the Mackie MR-5 speakers with 5" cones and tweets......and have a Polk audio sub with 10" cone and somewhere around 100W.
The critical thing in either using a sub or not using a sub is to have your system balanced and YOU need to understand it's strengths and weaknesses in either or BOTH configurations.
People with small cones and no sub tend to mix bass heavy muddy sounding mixes, whereas people with subs can easily mix bass lite mixes.
It all comes down to knowing the system. It depends greatly too on the style/genre of music you mix. Some genre's of music.... hip hop, rap, electronica, for example have really low frequencies....and the small cones simply can not reproduce them accurately. You can't mix accurately what you can't even hear.
If you are planning to invest in, or have just purchased a sub for the small studio, you need to have it set up and balanced properly. Part of that relies on an unbiased judgement call from listening to prerecorded material through the speaker. Does it sound right? Too much or not enough bass or just right?
Another thing I recommend highly is a program called ARC. Automatic Room Correction software. It's super critical for ARC to work correctly, it needs to be set up and calibrated properly. Once that's done, the room acoustic's quirks are taken mostly out of the picture. Sound waves, and especially the bass frequencies can really be a problem. ARC isn't the cure to a bad room with out doing treatment, but it can certainly help in mixing to a fair degree. It adjusts the sound going to the speakers for your "sweet spot" so that it reflects the truth in the mix.
It has taken me quite some time to get my sub properly balanced and the entire system calibrated to where my mixes are reflecting what I'm hearing in my studio, when others play that mix on something other than my speakers. And yes.....I always mix with the sub on. I have it plugged into a power strip with a handy on/off switch so it is possible, if I want to check the mix for bass, I can shut it off quickly and turn it back on the same way.
My room, BTW has carpet on the floor, acoustic tile ceiling, and furniture in it so it actually sounds halfway decent even though it's not treated.
MY STUDIO : this is an older picture.... but the monitors and sub are still in the same place. Monitors on the desk...right and left... the left on is partially hidden by the music stand.... and the sub is on the floor to the right, under the guitar amp and the power amp..... front firing.
So... no, technically you do not NEED a sub to mix better bass..... however, it sure sounds good.....