There's no real pattern, it's just that in some cases I would opt for a Google product because it turns out to be cheaper than the competition or in some cases there was no equivalent.

For me the services I got annoyed with the most were Cloud Print/Domains/Latitude/Reader because they were actually decent. In almost all these cases there were no official Google alternatives provided, at least not immediately; and in the case of Cloud Print there really isn't an alternative other than a VPN connection which is incredibly hacky. It bothers me they are discontinued in the way that they were because it just feels disrespectful.

from my observations there are two reasons that Google products continue 1. they print money 2. features can be added

my favorite google product was google talk and suddenly when it had zero growth as it was perfect. I would have loved to have paid for it to keep it available.