I think the basic premise of the post is accurate with regards to legalism. But we need to define our goals with regards to things such as this. Is our goal to mock legalists or attack them as people or to bring them into the proper doctrine?
Statements such as this give me pause.
The implication and tone here suggests that all people with legalistic tendencies are doing so in order to destroy fellow Christians' joy instead of just following what they think is proper doctrine. Should their doctrine be corrected? Absolutely. Should all legalists be lumped in together as people who "slither in the side doors of churches?" For my part, I certainly think not. We get on forums such as this and a lot of us have these well staked out positions that we've had for years and it becomes easy to think that everyone has well staked out positions that they've had for years and that they're willing to argue for hours over them without really listening to anything that anyone else has to say, because a lot of debates between people who have well staked out positions go that way. Pews are full of people who accept the doctrine of the people teaching them and that, just given a simple loving explanation of why that doctrine may be incorrect, are willing to drop it in lieu of the truth.
One thing I'm pretty sure of is that approaching someone with tones such as this in an attempt to correct bad doctrine is going to put someone on the defensive immediately and make it all the more difficult to show them the truth.