Slughorn had a specific motive - he wants to find those naturally talented students so he could form more connections with those wizards and witches most likely to become powerful in the future. To do so he not only needed to present difficult tasks, but also suitable motivation. The boring but complex potions will hardly generate the interest that a love potion would generate, and it might be enough to bring the cream of the crop to the top.
Even if that weren't the case, education often teaches hard concepts one may never use in order to demonstrate principles, and strengthen the foundation of one's understanding of a topic. In the same way that understanding a complex and difficult recipe can help a chef prepare simpler recipes when called on, understanding a difficult and complex potion will make understanding easier potions - and creating them from scratch - possible. You may never use calculus as an adult, but you know that a pool with a sloping bottom will fill more slowly as it nears the top despite the same rate of water input, and you'll know why.
Further, magical practitioners also need some experience with the more nefarious potions so they can identify them when needed, or avoid them if someone is attacking them in a hidden manner. This could be taught in Defense Against the Dark Arts, but it properly belongs in potions and teaches several things there.
I suspect this was also simply Slughorn's teaching method. While Snape focused on the foundational ingredients and preparation of potions, Slughorn discussed the high level concepts and results one could obtain.