I think it depends on the league, the rules obviously, and what is agreed upon in terms of kosher or not. I'm in a couple of keeper leagues where it's very common for someone to (temporarily) not have a kicker or a defense, or maybe even a TE if they're streaming that position. In other words, pick up the extra RB or WR on Wednesday, with the intention of it buying you an extra 3-4 days to decide which one to drop for that PK/DEF/TE on Sunday morning.
Many leagues have rules that make it harder to do so. Acquisitions cost $5 in one of my leagues, which makes it harder to stream. On the other hand, it makes it easier to compete for those of us who aren't afraid to spend $$ (half of the league are tight-wads).

Other leagues set rules where you must have at least one of every position on your roster at all times. Others have rules where you can't drop a guy you've just acquired within the same week. Others set a maximum per position (5 RB, 3 TE, etc.). I'm sure I'm forgetting others, but there are lots of ways to discourage what could be considered a mild version of churning, hoarding certain positions, or whatever you want to call it.
If the rules allow the empty spot (in other words, there is nothing that prevents you from not having a PK on your roster), I see nothing wrong with it. If people have a problem with it, they should ask for a rule change to be considered. Generally speaking, I'm of the "all is fair in love and war as long as it's within the written rules/parameters" thought process, but sometimes you have to use common sense as well.
Speaking of which... A few years ago, in one of my redraft leagues, someone tried to draft all players with a Week 7 bye. He was just planning to take a loss that week, with the theory being that his team would be stronger than the rest in all other weeks. The overwhelming consensus was that his strategy went against the spirit of the league's rules (and gave an obvious advantage to his Week 7 opponent), so we ended up adding a rule where you could not submit partial lineups (including bye-week players). You can still temporarily not have a PK in that league, but you can't submit a lineup without one.