I've seen some temp jobs work out well, but I've seen others where it was not so good.
Temp-to-hire where the employer actually really does intend to hire-on, and uses the temp-process to get to know candidates before making offers is fine. It's actually not a bad idea if basically everyone is on the same page. Temp agency needs to be ready to move people around if various employers do or don't like candidates, and temp-employees need to understand that there could be periods of downtime, and might themselves need to ask the agency for alternate placement if they don't like where they're temping.
On the other hand I've seen temps that were abused very heavily, because regular employees didn't want to do shit-jobs or didn't really want to work at all, with no intent on actually hiring. I've also seen rather odd people working as temps because even in a temp-to-hire arrangement the business didn't like some of the temps but still needed work to be done so kept them around for longer than normal just to complete the task before releasing them.