Is it a good idea to upload rejected papers in arXiv?
Yes, you should upload them. 

If you think the works are good and worth reading, but don't see a good opportunity to publish them in a journal or at a conference, then the arXiv is the way to go. Actually, many people have unpublished manuscripts and even some influential work only exists as technical report or preprint. It's not uncommon. 

If it would be worth it to pursue "real publication" is a different question...