He has a torn ligament in his knee. His car was stuck in the snow. Probably not a great idea to do a lot of shoveling with his knee.
Belichick expects you to be at practice no matter what the circumstances are. Aqib Talib chose to stay at the hotel at Patriot Place, so that he didn't need to worry about such problems.
Spikes makes millions of dollars. He could have chosen to make any one of a number of better decisions to get to practice. Money goes a long way. I guarantee you that many people in the Boston area did not miss work, and they have less resources, make way less money, and have less of an incentive to get to work than Spikes did.
He could have paid someone 1,000 dollars to shovel the driveway, got up 30 min earlier and taken a cab, arranged for a ride from a friend, etc, etc, etc....
So, do you think they should have sat Collins in favor of Spikes, or had Collins start and play lots of snaps and all the special teams and carry Spikes on the bench?
We've seen plenty of players who have bit roles on the team before, particularly at the position of LB. He could have had his snaps managed or lessened. Most importantly, even if he did have a partial tear (which is pretty broad for a knee), he could have simply missed a playoff game and automatically rested 3 weeks from the end of week 17, or he could have been rested for the last regular season game in hopes of getting better.
Nothing in the world suggests that Spikes was struggling that badly to even miss practice. As I stated, the injury situations with Dennard, McCourty, Mankins, etc were all managed properly during the bye week---yet Spikes did not even miss practice.
You keep stating how bad his knee was, yet it goes against all logical thinking to assume that Belichick was not managing it for a postseason run the same way that he normally does with many other players.