I recommended the cube because it will take the beer off the yeast, proteins etc and if you leave it at ferment temps for a few days after fg, the yeast will have done its job. Risk of autolysis is pretty minor but I have found that extended contact can lead to some undesirable flavours in the finished beer ( meaty, yeasty, etc). This is minimised in the cold so in fermenter would still likely be ok.
Other reason for the cube is you can seal it. Unless your fermenter is sealed completely with bugger all headspace it's got some risk of oxidation. Not huge but some. Co2 is no longer being produced and while it might continue to come out of solution (temp dependent) it won't be as protective as the blanket formed during active fermentation. Oxygen favours things like acetobacter.
Infection is a pretty small risk if your techniques are good.
Finally cold conditioning has effects on dropping stuff out of solution which makes beer taste better and cleaner, style dependent.
A clear looking stout tastes better than a yeasty, murky one, beer matures with time and cold retards staling reactions.
Your fermenter for 3 weeks on the garage floor at this time of the year for 3 weeks will likely be fine - cubing and cc'ing is just what I would do prefetentially.