I wouldn't say a lot of units explictly NEED their skill points to remain effective, nor are their skills required to be high to manage most of the game.

Units that are a great investment, aside from those that you want to SAW, are the ones that need their skill in order to remain functional. Solano is a good example of this as she only attacks when her skill is up, thus max skill is a great thing in order to make her usable longer as well as have a shorter downtime. Angels also fall into this category.

Gold units in general though might not be the first thing you should focus to invest in SUing. Plat units with good skills and blacks are the first ones you might want to focus on, especially if you are going to use rainbow fairies. Sure, the extra some golds like Bernice and Iris get from their SU's might be nice, but they're not exactly gamebreaking unless you seem to have a distinct problem in that department.

That said, if rainbow fairies are on the table, consider saving them for the future instead of using them instantly. Extra copies can be used at your own discretion.