SEO is sometimes a bit of a balancing job. One example is balancing the competing desires to 1) rank well, and 2) have as many positive pages appear in the SERPs (Search Engine Result Pages) as possible for your branded terms - preferably from your own sites.
The issue is that each site gets only two results in the SERPs. So if you have three strong pages on a particular topic, only two of them will appear. One way around that is to use subdomains. For example, if you do a search on New York City College of Technology you'll see many pages from the cuny.edu domain. 6 of the top 10 are from cuny.edu. They got more than two listings because the pages come from a variety of subdomains - www.cuny.edu, www.citytech.cuny.edu, library.citytech.cuny.edu and eportfolio.citytech.cuny.edu - each subdomain is treated as a separate site and is given a limit of two pages in any particular SERP.
The problem with subdomains is that each "site" has less power, so your chances of ranking for a competitive term are lower if you use subdomains, though you can theoretically get more entries in each SERP, so it's a balancing act - power on competitive terms vs. prevalence on non-competitive/branded terms.
Another factor to consider is that secure pages are considered a separate site. If you have the same URL accessible via http:// and https:// you have a duplicate content problem. However, you can also possibly get more SERP listings that way... Take this search result as an example:
Notice that there are three listings for www.paypal.com, but if you look really closely you see one of them is https://www.paypal.com - a secure page.
So just remember that secure pages are their own site. Make sure you take care of duplicate content issues and see if you can use the additional "site" to your advantage for more listings in certain SERPs.