I am a big proponent of Tier 5 quests, but I can understand that the resource allocation and development time is too great to justify making more.
I can't say whether or not only 10% of players use Tier 5s, but I'd like to hear specific reasons as to why you'd avoid clearing them. In my game, the Tier 5s available so far have been the apartment building (9 out of 10 times), and sometimes the hospital. Though I suppose you can always get assigned one of the brand name factory POIs, which are certainly more difficult. There are roughly 5 traders that I run in a circuit for quests, and often times I get 2-3 quests at the same building. The best part about the quest system is that the POI completely regenerates once you start the quest, and Tier 5s are offered 100% of the time at the trader once unlocked. Essentially, you get unlimited kills for exp, and unlimited salvage and loot... if you know where to look. On top of that, especially if you put enough points in daring adventurer for +2 reward choices, you always get at least one 6 quality tool/weapon, and if all of the rewards you're offered are not upgrades to your current gear, just sell the most expensive reward for dukes.
Speaking from experience playing on warrior day 50, though I have been clearing T5s since day 20. Considering every single potential spawn, there are maybe four enemies that are a legitimate threat to the player. One of them is the zombie bear. The other three would be the radiated biker, radiated cop and radiated soldier, which spawn mostly near the loot room if they spawn at all. If you are in a large party, your gamestage is going to be through the roof, and its highly likely you'll be spawning radiated tough guys even on the regular floors of the POI. The positive is that nothing beats the fun of team gameplay, even if you run a stealth build and no one else on your team does.
Playing on an agility build which is really OP for quest clears (only if you're solo, lol), I can run the easiest Tier 5, the apartment, in around 25 minutes depending on how little I loot, using mainly a compound crossbow with iron bolts and a silenced pistol. I opted to use Advanced Muffled Connectors on all armor along with rank 5 in the sneaking skills to stay absolutely hidden. And I'll throw in ghillie suit legs as an added bonus. I would estimate that there are 150-200 potential spawns in the apartment, but hardly any get the chance to wake up. If I can proc the sneaking bonus on a headshot, everything dies in one hit besides big ferals/radiated. It's like shooting fish in a barrel, except the fish don't even move when you line em up.