this is a difficult issue. Restricting the number of inventory items too much can mean that players won't be able to pick up items before they can actually use them, which leads either to the "I don't want to take that now (but later)" problem, or to miraculously added hotspots. This would be bad design, IMO. It would make the game feel more linear and restrictive than neccessary.
Limiting the overall number of items would mean that you'd have to drop items to pick up others, which may be "realistic", but in reality it just means a lot of backtracking, which is unpopular, and justly so.
MOS tries to keep the number of inventory items reasonable, and you won't carry ladders or other ridiculously huge stuff around. Of course, Peter's pockets are still too small for all of them, but:
a) we thought he'd look silly with a rucksack
b) for obvious reasons, we can't have a chest follow him around like in Discworld 2
c) I've played XIII recently. I carried six or seven different handguns and rifles, and loads of ammo with me. Where did I put that