What do you mean? A 60 year old lady would be safer in the main traffic lanes than in the gutter, for exactly the same reasons as anyone else - more visible from behind, more control over when cars pass them, no risk of hitting a car door etc.
Cyclists stick to the right of the road exactly so cars can pass them safely.
Anyway, I'm out of this discussion, I find it very interesting that commenters from countries where cycling is decidedly unsafe are going to lecture those from countries where the predominant mode of transportation is the bicycle on safe cycling practices. If the US would adopt half the measures of what we consider normal here when it comes to roads and other facilities for cyclists and cyclists there would adopt half the practices of what we here consider safe cycling then you might have a chance at making cycling a viable means of transportation.
In the meantime, good luck with 'assertive cycling', occupy that lane, it's yours after all and make sure you wear that helmet, you'll need it.
This is exactly the point though. The guidelines that apply in the Netherlands are different to the guidelines in, say, the UK, precisely because the two countries have very different attitudes towards cycling. In the Netherlands you have an environment where cycling is extremely common, motorists are very used to sharing the roads and in fact most motorists are also cyclists themselves. This dramatically reduces the sort of us vs them conflicts between cyclists and motorists that seem so common in other countries. Different types of road user are seen more or less equally and show each other a level of respect that is just about unheard of elsewhere. In such an environment, hogging the lane seems unnecessarily aggressive because you simply don't need to do it.
In the UK, USA and just about everywhere else motorists are simply not used to driving close to cyclists. They don't know how much space to leave, they don't know how to pass them safely, it's actually socially acceptable to be aggressive towards cyclists but most of all, motorists are not on the lookout for cyclists, they are not expecting them. In this kind of environment the cyclist's number one priority is to make themselves seen and that's why being assertive is so important.
If you're in the middle of the lane by default the car behind you might be annoyed but at least they are likely to see you, and when you've been seen you can then move closer to the kerb to allow them to pass you safely. This puts the cyclist in control in a situation where they'd normally be at the mercy of motorists.
We are lecturing you on the optimal way to cycle in /our country/, not yours. Sure, your country might have a thing or five to teach us about infrastructure, laws, and cultural norms regarding bike traffic, but when it comes to what an /individual bicyclist/ should do given the situation here, we absolutely have a better idea than you do. It's like comparing driving in a easygoing city and driving in an aggressive city--sure, the driving in the former is on the whole safer than in the latter, but driving in an easygoing manner in an aggressive city is much more likely to get you into trouble than driving aggressively will.