Any point in powering up Golem as a defender

Golem makes a terrible defender because of his awful typing. If you want him strictly for defense then there is no point, his CP is not bad but Rhydon with the same awful typing has a higher high CP and can be placed higher in gyms.

If you have a lot of candy, dust and just want a golem for variety then go for it. But be warned that he will be very disappointing defensively and offensively he is not necessary at all.

I certainly wouldn't power up Golem as a defender. Same with Rhydon, they're just not good defenders.

No.
Double weaknesses (x1.56) to water and grass makes pokemon useless as defenders.
(While Ice, being a "rare" type, lets Dragonite as a much preferable defender).