In private beta, the aim is to quickly get many good answers and good questions.
Does the daily reputation limit hinder this? If I'm at my limit, I might choose to wait until tomorrow to post more good questions and answers.
There's more to a private beta than just generating a lot of questions and answers. Another aspect of it is to attract active users. If you hit the rep cap and decide to hold off till tomorrow to post something (though you shouldn't :)), that gives others a chance to contribute instead.
So it all balances out in the end.
That said, it's rare for someone to consistently hit the rep cap day after day during a private beta. If that happens, that would suggest to me that something's possibly a bit unusual on that site. For example, either the questions and answers are too obvious, or maybe there is a user who's "carrying" the site. It's not necessarily a sign that something's horribly wrong, but it'd draw my attention.