I don't. Absolutes keep things nice and simple on both sides of the screen. As soon as you put it on any sort of sliding scale of penalties, though, players will gradually find ways of expanding the borders and-or reducing the effects of those penalties over the years.
Picking locks is not the only thing a Thief does, though, and the restriction is made blanket to avoid the alternative: having to define and narrow down which armours can be worn while doing which different activity.
I mean, you could theoretically pick a lock wearing field plate, except you'd still have to remove the gauntlets (to free up your fingers) and helmet (to see what you're doing). That, and the way I see it heavy armour should affect your overall dexterity somewhat, and your maximum move speed unless you're crazy-strong. 1e doesn't have this - in 1e full Dex bonuses to AC apply no matter what armour you're in - but it's never made sense to me that someone clad in 60 pounds of metal can move as freely or as quickly as someone in a leather jerkin.