The only times I've ever taken out loans in EU3 was when I had just made a major cash investment, and then got hit with a random cash-sucking event right before the end of the year. I remember one event which hit on the last week of December, and I had to take out the loan to cover it, knowing that I'd have had enough to cover it if it hit one week later. With several events, not scrounging up the cash will cause some serious damage or long-term harm, so taking out a loan is the least-bad option. In at least one case, the loss of money could not be avoided, so taking out the loan was automatic.
Of course, at some point in the game (with the DW expansion), just about EVERY minister you can hire will be a Banker, to reduce loan interest. By that point in the game, I'm generally sitting on a pile of money that I can't spend due to lack of Magistrates, so they're 100% useless. Take your pick, any one of 20 different 3 star Bankers, or another 10 with 2 stars, all looking for a job, or else you can hire that lonely one-star Navigator at the bottom of the list. A couple of decades earlier, it will be all Theologians. Later, it will be all Natural Scientists. I was really unhappy when the expansion broke the random selection of ministers, and tried to impose what it THOUGHT you would need at that point in the timeline, to the exclusion of practically everything else.