Should you only use engagement programs if you plan on always adding more email content meaning it essentially is an on-going program?
Not necessarily. I find that there are many other benefits to EPs like being able to turn them on and off, expire content, rearrange content, prevent a person from receiving the same content multiple times, and more.
Would your opinion still stand if you knew that you never would add more content to that program after a series of 5 emails went out for instance?
If you were never planning on adding more content then I would be comfortable creating as a drip campaign. Especially since depending on the size of your instance/org, there are a limited (I believe it's 100) number of EPs that you can have. But if you don't want any of the benefits that EPs provide, then a drip would be totally fine.
EP's notify you when leads have exhausted all the content in the stream. Plus each email in the series gets an Engagement Score.
In addition to all the benefits mentioned by Dory, I find EPs especially convenient when you want to add content later one. Doing this in a standard flow causes a lot of headaches with leads that have already completed the flow or leads that are in wait steps when you want to add some content in the middle of the flow. Same if you need to change the order of the content.
In EP's all these operations are immediate and easy: just drag and drop your content, and let Marketo do the rest. It will handle who has received what on next cast and this will be done.