My thinking is that any startup that has got to a state where they're ready to collect/offer beta or pre-launch registrations/invites ought to have all the resources to offer this on their own site.
So, why use third-party services?
1) Someone with no coding skills can put up a nice looking page without needing a designer or programmer. Market testing an idea then becomes a lot cheaper. This happens before there is an actual "startup" with a dev team.
2) It could be cheaper/easier than building it yourself. From a managerial perspective, this could make a lot of sense. Option A is taking your dev team off the core product so they can build a landing page, integrate with MailChimp etc. Option B is paying a few dollars per month and having it done in an hour or two.
Further to your reason 1)
It is perfectly understandable for cases where people just want to 'market test'. What my question really concerns is not such cases but those that are more or less ready with their actual service and still use third-party landing page services.
I mean, won't it be like loosing good 'branding' opportunities for such startups?