I agree with the IKEA affect, at some level, but believe people are wrong about what it means. Just because someone has worked on an open source project does not mean they have rose colored glasses and expect it will solve every problem more efficiently than another alternative. In my view, it means that they have a more sophisticated view of what the project actually can do, in part because it is open, and are ready to share that information.
I own an open source company that deals with Drupal and CiviCRM. It is not uncommon to be in a conversation where someone is telling me of course I think Drupal is the greatest thing out there, and assumes I am not well versed in anything else. I can go on about the virtues of Drupal all day long, but that is besides the point. I have an in-depth understanding of Drupal, Wordpress, Joomla (and it's predecessor Mambo), Plone, Xoops, and a number of other open source tools.
I have contributed to each of these platforms at one time or another and understand the way they work in great detail. Compared to Sharepoint, where I don't understand the internals, I am not going to have a lot to say. If you come to me asking what you should be using, I am going to talk about Drupal, but am also going to ask what you currently use.
There are parallels with the automotive industry that can help explain what is going on there.
My mechanic actually makes cars. He purchases transmissions, chassis, all the component parts you need to assemble them. He has a large lot, looks almost like a junkyard on the outside, and he keeps a fleet of jumkers around to restore them and sell them off. On the inside, his shop is a paradise of tools, diagnostic machines and the like.
His obsession with cars extends to his personal life. His house is filled with cardboard boxes that contain custom parts he picked up because he knows what he can do with them. He can explain them in terms of torque, output and a lot of other factors that go beyond my ability to appreciate a car and what it does.
My neighbor is also someone I would call a car guy and drives a german supercar. It was top of the line when he bought it. I mention the car to the mechanic, and he can tell me about every part in it and why it is good or bad. He has strong opinions about the car and why it is poorly designed, with several prognostications about parts that will die prematurely due to flaws.
When I speak to my neighbor about it, all he knows is he has an expensive car. that impresses people. He can talk about all the luxury lines and his knowledge of the component parts extends to the makeup of the interior, the warmth of the seats, the placement of cup holders, and the like. What he really cares about is not under the hood, it's how the car looks to other people.
If you put the question to both of these kind of people about what kind of car to drive, you are going to get very different answers because one understands how cars are built, the other understands what the car means to other people who see it. There is a qualitiative difference there people don't always appreciate between different types of afficinados.
That said, there are ideological zealots out there who will always tell you to use a platform for it's own sake. I don't always get the sense these people always know what they are talking about, and generally get the feeling they cling to one platform due to their ignorance of the benefits of others. They have a tendency to become very defensive when confronted and make very bold assertions in the absence of facts.
This later class of people generally don't have much to do with how the platform is built. They tend to be the ones who are proponents of the platform and have strong opinions based on their participation in the community. While they can be fun to spend time with, there are situations where you get sick of being around them. To be candid, there are a lot of people in open source communities who are like this, and I think that's where the confusion comes from.
But don't mistake them for the people who actually love open source and understand it's benefits and drawbacks in comparion to other platforms.