These two quotes sum up the subject for me. Gift or not / rare or not, it doesn't matter. I feel toys are to be enjoyed and if assembly helps you enjoy them more then go for it. Collectability be damned. Now saying that, getting a blank 'customised' by some 'painter' is another matter I might not be so quick to agree with!
Blanks are way more fun anyway. You can pose them without risking paint rubs. Take fun pics without being paranoid about the toy tipping over. Even bust out the blow dryer and do stupid mash ups with other blank toys.