The ONLY reasons to use systemd timers rather than cron are:

1. When you can accomplish something with them that you could not accomplish with cron. There are several such things. Being able to accomplish them is a good thing.
2. If you want to eliminate cron entirely from your system for whatever reason. One reason might be that the blasted thing wakes up every second to check its jobs and then goes back to sleep, which is not the most power efficient behavior. I suspect but have not verified that systemd is rather more intelligent about waiting for timers to fire. (It wouldn't be too hard to fix cron to sleep long either.. too much horrible crufty C code for me to do it tho.)