It is of my opinion that in most cases having a schedule (every day at 8:00 AM, for example) helps keep your subscribers engaged as there is a level of expectation. And, the expectation helps with open rates for which helps with engagement. Also, if your audience goes 3 weeks without hearing from you, how much churn do you think you will have?
I do believe, however, that in some niches, spontaneous is key, but this is my exception; not the rule.
And, as always, quality over quantity.