To RAID or not for me depends on <1> my budget, and <2> whether it's critical to have real-time redundant data. IME, inexpensive / consumer-level RAID controllers aren't terribly reliable, which means I'd have to budget more than I'd like for a HQ RAID card. And personally, my critical data doesn't change frequently enough to warrant (real-time) RAID, so implementing RAID would be overkill. So instead, I employ
reasonable redundancy without RAID, in my case mirrored on a nightly basis. I'll never lose enough data on a daily basis that it will cause undue harm or inconvenience. Others may define a different period between mirroring, like 12 hrs, 8 hrs, 4 hrs, etc., depending on individual needs.
Regarding your concern about deleting a file, for example - that's what optical / tape / off-site backups are for. Redundancy simply allows swift recovery from a single-point failure. Don't count on a redundant HDD system as your backup - it'll cause you great pain, eventually.
Edit to add: Free FileSync (referenced in the Reasonable Redundancy link above) allows one to configure it to maintain files on the target / "mirrored" HDD even if they're deleted on the source HDD. This means that if you accidently delete a file, it'll still reside on the target / "mirrored" HDD. The downside to using this option: it could potentially eat up a lot of space, eventually.
Edit to add again: Recommend having at least 3 separate instances of your data. I have my redundant HDDs, optical backups, HDD offsite backup, and for most recordings also an offsite backup in the form of music fans who've downloaded my recordings.