and then we save the game and eventually we will be forced to dismissed her because there's a quest that needs certain npc to follow us or you want to try other followers...at this point you can still recruit her,BUT,if you load the game any where and at any point in the game,you will not be able to recruit Illia again,even though she can be found at her initial place (Blacklight Tower)
it seems the save game triggers the bug..and so i came up with an idea to change the save game file name..and it worked like a charm!!!she can be recruited again,huraaaayyy!!!
okay here's how to do it:
1. complete the quest at Blacklight Tower (repentance)
2. ask her to join your side (best if you don't have any follower while doing her quest or dismissed anyone before the final battle)
3. save the game with a console command so you know where to find them later on, ex: press "~" and then type "save (yoursavegamename)" without the quote and the bracket..let's just use "save test"
4. ask her to leave the party and check again if she's still available for you...it should be,hehehehe...so now there's only you in the group
5. save the game via menu and replace the existing file (your previous savegame file on number 3 in this guide)
6. load the game and ask her to join your party,at this point you won't be able to interact with her,and she will just saying any random words...
7. exit the game,go to your save game files on my documents/my games/skyrim/saves and look for "save test.ess" maybe there's "save test.ess.bak and the others...but right now stick to "save test.ess"
8. rename the file to "save test1.ess" for example
9. load the game and ask her to join again,and voila!!!!she can join your cause again,hehehehe...
NOTE:remember,everytime you seperate with her you have to change again the savegame file name and proceed to recruit her
i have tried this 5 times,i hope you guys can have the same fun as me though,hehehehe

btw,sorry for just temporary fix as i can't wait anymore for the patch T_T