I have heard long ago that to learn a song in all keys was beneficial and would make one a better pianist. Is this true or not? I have never heard the reasons for this. If beneficial I plan to pick a song and do it. However, it will take time, much time so I want to be sure I am learning to do something that in the end will make me a better pianist.
On an acoustic piano, it's a great skill to have, being able to play proficiently in all keys. Playing with others will make this easy to understand. Just being able to play a particular song in one key is quite restricting when the singer asks for it in a different key.
Having said that, I occasionally play with a keyboardist who has an electronic piano, and he happily uses the transpose button to play everything in C on his instrument. He's a nice player, but I can't help thinking how his playing would be even better not merely using those same old notes. In fact, I sometimes use the transpose when the band's played 3 numbers all in A - so I can use a different set of notes to stave off possible boredom!
Taking your time to learn a piece in different keys will give you an opportunity to practice your proficiency playing in each of those keys. Do you find that you have a problem playing in certain keys? If so, this could be a useful exercise.
Another skill that you may (or may not) find useful - depending on the settings on which you play - is the ability to transpose a song 'on the fly'. For this skill, taking your time to learn a piece in different keys will (as before) help your proficiency playing in each of those keys, but that's not enough - you need to also practice taking a piece you know in one key, and then playing it in new key unprepared.
Of course if neither of those sound like skills you need to learn, you might find your time is better spent doing something else. (As Tim alludes to, the second one may only be as useful as knowing how to use the transpose button on your keyboard!)
If you ever play in a band or accompany a soloist, you may have to change keys to fit the soloist. Especially if the soloist didn't get much sleep the night before.
Practicing a piece in various keys allows you to concentrate on how a pattern feel (or look or sound) in different keys without having to learn many different pieces. Not that learning other pieces isn't useful too.