The only possible problems I can see really, is the child loosing the pacifier during the night, and not being able to find it to get back to sleep, and you having to get up and sort it out on a regular basis.

Some poeple will tell you it can affect teeth development, but I myself had a pacifier at night 'till really late (i think I was at least 4) and my teeth are perfect- my youngest sister kept hers even longer, and her teeth are fine too.
Mind you, the sister in the middle was never big on pacifiers, and was a thumb sucker- her teeth were not the best but they've straightened themselves up considerablely since she's become an adult.

It's also alot harder to stop a child from sucking their thumb- it's not like you can take them away!!

I guess there is a remote chance of choking if the child tries to swallow it maybe, but i've never heard of that happening.

If you're happy, and your child is happy, let him keep the pacifier.
It's not like he'll be wanting to take it on his honeymoon or anything!!!

It may soothe him now, but he may not be able to sleep without it when he grows older. This could turn into a problem when you try to get rid of it. It is probably fine for now, but you may want to try getting him to sleep without it in a while, so that he can learn to soothe himself without depending upon the pacifier.