1.  Maybe but not according to some state laws...  Enable encryption, and in most cases NEVER worry about being in the news about lost data.  In about 20 states, encrypt your data and lose it, you dont have to say a word if the lost data contained personal customer records.  

2.  Thats compression if I recall.  But you do have choices in AES 128, 256 etc, which are varying levels of encryption.  Obviously some are more secure, and some dont take some 10,000 years to crack and decrypt.  The more secure, the longer it may take to process the encryption algorithm  There is always a trade-off...

3.  Currently BESR2010 is backward compatible with BESR 8.x images.  I can't speak for older images.  That said, there will always be some level of backward compatibility at least 1 major revision behind it seems based on historical changes in BackupExec products.

thanks for your answer, but i still have some questions:
 my first question wasn't legal. what i'm asking is if there is a difference between using an encryption or not ,if in both cases i use the same password. does the encryption makes the data more secure ? after all, we still need the same password (maybe i'm asking this question because i don't know anything about encryption...)
about my last question, and i think i'll need a Symantec support member, are there known cases in which even after using the correct password, still the data couldn't be decrypted ? and what i'm supposed to do with my current backups in the future ,if i want to upgrade BESR to a version which is not compatible with the current encryption method ?
thank you.