as i know that google ban the directory sites because of the bulk spam posting but now this threat on the book marking site because considering the book marking the as the spam may be in near future google ban the book marking site as well

If you really want to do bookmarking without getting marked as spam then dont cross the limit, just find 20-30 high traffic bookmarking website, participate in community, be active and post other stuff as well.

It is benificial for you if you bookmarking on sites with different description becuase same descriotion in every bookmarking sites may consider as a spam.

Well, I guess you have spotted the right thing bookmarking sites are also sometimes treated as spam. Google still says that its okay with the social bookmarking but if you talk about overdoing doing bookmarking its again considered as spamming. Here I would say instead of moving with things in bulk try to remain over the limit overdoing things will always get you over the hands of spamming.

Google didn't ban relevant directory submissions (dmoz, yahoo directory, etc. are just acouple that come to mind). What they did ban were the directory submissions where people were submitting to spammy sites or in irrelevant portions of the directory itself (ie. posting your website about dogs under the directory's computer software section).

Now back to the topic of social bookmarking. First and foremost; search engines love anything thats "social" because they can better analyze the popularity of a website. Over time they probably won't ban social bookmarketing they will just turn it into a low ranking factor (ie. <1%) if it does become spammy. However again, if they are a reputable website with a large community (ie. stumbleupon) the community will weed out the rift raft relatively quickly and only good quality websites will continue to gain advantages through this SEO tactic.