For defense, Lapras in gyms depends on how high the CP is for your area...I have 2 Lapras and don't really use them much for defense since strong gyms in my area are 2700+ now. Need to power up probably to Level 35 or so to have staying power so not as useful sadly.
If you want to attack with it and you have no other ice type mons, you can power it up, but I'd guess a level 20 Lapras can destroy any Dragonite in gyms anyways so you can test it out and see if it's necessary for you.
That said, it IS a Lapras so for their general rarity, it can't hurt just for fun and general use to power up. It did take 5 months to get one so who knows when another will come along.
If another did, you can power that one up to.