probably, but 5 minutes per install quickly adds up if you need to install 100 / 500 scripts. And since it's free, users tend to have various odd requests as well, which adds to the free admin time. This is fine for a hoby driven site, but not for a business who pays a salary for a tech todo free work.
From a business point of view it makes a lot of sense to have an option where users can easily install common scripts like CMS', Blogs, Wikis, Forums, etc from a script installer. Fantastico on cPanel, and Plesk Powerpack does this and people pay for those control panels because of those features. I'm sure such a feature could increase ISPConfig's popularity as well