I think it's almost pointless to have "views" on things like this, as so much will depend on circumstances.
I think on your first paragraph, people are actually pretty foolhardy not to get married in those circumstances. Yes, marriage is a demonstration of your commitment towards your partner, but that's only once facet of it. The other part is legal protection. Simply living together with children gives you pretty much fuck all legal protection if the relationship breaks down. There is no "common law" marriage, so if one person takes time out of work to look after the children (not even on a permanent basis, but say 18 months per child - that's still 3 years), then that goes completely unrecognised in the eventuality of a split.
Whether or not you're married, that person has still spent three years total out of the work force. That's three years less experience, three years less promotional opportunities, etc etc. It gets a thousand times worse if one person is a stay at home parent.
I can understand why people might not want to have religious ceremonies, or a "traditional" wedding, or a big song and dance. However, to skip marriage entirely, and all the protection it brings (not to mention tax breaks in some countries) is pretty incomprehensible to me.
With long engagements...as I said, so much depends on circumstances. People might get engaged with the intention of marrying the year after, but then discover that the venue they set their heart on is booked up that year. Or that they need a little more time to save money - there's nothing wrong with wanting to wait a little longer so you can afford to invite everyone you want to. Or to have whatever niceties the couple have set their heart on.
Or perhaps people get engaged and then personal circumstances mean it's difficult to focus on a wedding (death in the family, job loss, etc). Or maybe they get engaged, then the bride gets pregnant and they decide to have the baby first so they can both drink at the wedding/the bride can wear whatever dress she wants/insert a million other reasons here. There's nothing wrong with any of those choices.
I do agree though that there are some people (mainly men, but I guess some women too), who get engaged as a way to keep the relationship "on hold". They don't want to get married, but aren't sure if they actually want to break up either. So they get engaged and then drag it out for as long as possible.
In those situations I think it's extremely manipulative and unfair.
I also find it confusing when people get engaged, and just stay engaged for five years without ever planning anything more (for no apparent reason but you never know what's happening behind closed doors). In those cases I think it's partly a matter of wanting the social status of "being engaged" without the annoyance of actually planning a wedding, or having to sign up to the legal reality of marriage.