I guess, shared RNA/ DNA pieces that are present in your RNA sample may participate in the RT priming reaction. you can perform your reverse transcription without any primer and you will get some products. However, using dT, you will get significantly more products from A-tailed molecules, but as rRNA genes are so abundant and actively expressed, you can obtain also some unspecific products with these sequences...

Hi Qianqian Zhang, This is not really true to all eukaryote organisms. For example, Leishmania parasites present polyA tailed rRNA which could also be enriched when using oligo dT methods. You can use the enzyme 5'-terminator, which is a 5'-phosphate dependent exonuclease (epicentre - http://www.epibio.com/enzymes/nucleases-glycosylases-dna-binding-proteins/rna-exonucleases/terminator-5--phosphate-dependent-exonuclease) prior to your PolyA+ selection. This enzyme cuts 5'-monophosphate RNAs which is the case of rRNA. Bi or triphosphate 5' are protected. So, it means that you can reduce your huge amount of rRNA before your cDNA synthesis using oligo dT. Good luck!