Hi, you'r welcome.
No, all the synonyms and antonyms aren't useful and you shouldn't memorize all of them. The ones that are bold and have larger size are GRE words too (are in the list) so I think one should only learn them. For example, look at http://www.killthegre.com/words/view/abate in the list, the following synonym words are GRE words and are in the list:
subside, allay, quell, subdue
You can click on the GRE words on the synonyms and antonyms section. By learning the synonyms and antonyms of a GRE word one can learn faster because in this way one divides the words in several groups and each word in a group has similar meaning.
Please note that to see all the synonyms and antonyms of a word you should click the word on the list (on the worldlist, for each word, 10 synonyms and antonyms are shown at most)