Running 1.50 on the slim is a POC for testing 1.50 kernel brew mainly. 1.50 is not useless by any means, its the only firmware that has kernel mode (yeah 1.00/b). This is the main point of DC3/4 needing a 1.50 subset. All 3.XX brew is user mode, for 3.XX kernel mode you need to load all the kernel calls via a external PRX.

Now that Sony keeps changing the NIDs, and more so completely randomizing then, makes it very hard to find them, if not imposable. If the slim could run a kind of 1.5 kernel the NIDs for brew would never need updating thus making the brew never needing an update for newer firmware versions and keeping legacy.