That's the point. There's perfectly fun and viable playstyles that don't need any points in pack mule at all. Doing without _any_ of the perks is a bit of a challenge.
You want to rack up rewards by running errands for the trader then sure, put points into pack mule, load up on everything in sight and ferry it all back to the trader. Just don't complain that you can't do that _and_ other things all right from the get go. Choices.
Building big doesn't need inventory slots. I know, it does if you play as you do, so choose that path and let others choose other paths. I can't be a kickass bow sniper _and_ three-hit doors _and_ build a forge _and_ run all over the map _and_ hoover up everything in sight all in the first week, not on stock settings, is that bad?
I think maybe the game difficulty settings could alter stamina regen and encumbrance slots along with nerfing the zombies, there sure do seem to be a lot of players who regard having to choose as a bad thing, if they're so ragingly mad that the game forces hard choices on them and you can ease off on those players by tweaking rates or limits, well, why not? Change the difficulty level names, "It's a Wonderland!", "Hey, not too rough!", "Okay, Hurt Me.", "It Hurts. I Like It.", "Well You DID Say Apocalypse"..
