keeping the SP3 plugged in will not hurt the battery, but it IS recommended that you let the SP3 drain down to 10% once a month to keep the Battery optimal
I would use it more than once a month. I would try and use it a few times a week at the minimum on battery.
I ruined every laptop battery I have had in the past from MacBooks and MacBook Pros to HP laptops because I rarelky had a use to use them on battery power. I did try and use the last MacBook Pro on battery power at least once a week and after a year, it would shut off at 50% and running it down till then several times would not fix it or calibrate it.
I bough the SP3 because it's different and very easy to carry around the house. I don't travel and have no excuse to really take it anywhere so it will always stay home.
Microsofts guidelines listed above are assuming that you use the computer on battery power frequently and don't leave it plugged in constantly.