Experiment in your own dev or demo app with a web and a worker dyno for free. Sleeps after 1 hr of inactivity. Active up to 12 hours a day. No custom domains. 512 MB RAM.
Run a small app 24x7 with the Heroku developer experience for $7/dyno/mo. Custom domains. Run a maximum of one dyno per Procfile entry. 512 MB RAM.
Build production apps of any size or complexity. Run multiple dynos per Procfile entry to scale out. App metrics, faster builds and preboot. All Hobby features. 512MB or 1GB RAM. $25 or $50/dyno/mo.
Isolated dynos for your large-scale, high-performance apps. All Standard features. Compose your app with performance and standard dynos. 6GB RAM. $500/dyno/mo.
All dynos come with the core features of the Heroku platform: Git-based deployment, managed patches and updates, automatic dyno restarts and aggregated logs.
I agree about the Free option being not as useful.
If someone wants to start on heroku, there is no risk giving them 24 hours of service. Since the money will still be yours when they start getting serious.
By only giving them 12 hours, you are missing out having them start with you, let alone staying.