Strategically, there's a major problem with giving a player immunity... You are not giving someone a "free pass", you are giving them money. Your money. And in a game where - if played as designed - money is in short supply.
Think about it - if you avoid the all-too-common "house rule" trap of putting $ on Free Parking, there's not a lot of money coming into the game system from the outside. Each player gets $200 per circuit of the board + whatever positve Chance/Community Chest cards they received, but those are balanced out by the negative cards & spaces (Income Tax, Luxury Tax). Money leaves the game at a fair clip as well: buying property, improving property & paying off mortgages all take money away from players and put it back into the bank.
Since very little money is coming into the system (it's not a zero-sum game, but it's in the neighborhood), you need to make sure that as much of that money is coming to you as is possible.
In game terms, "immunities" serve to make the game longer. Again, the game is designed to constrict - the amount of money available grows smaller & the winning player is the one who can pry his opponent's fingers off of what is left. By (essentially) giving players extra money (the actual consequence of immunities), you increase the time it takes to bankrupt someone.
Finally, the question of an official ruling on immunities. While there is nothing in the published rules about this, the rules used in tournament play forbid it. The following quote is from my own webpage on Monopoly at Game Central Station and quotes Phillip Orbanes' The Monopoly Companion.
- You may only trade Title Deeds, cash, and GET OUT OF JAIL FREE cards. You can't trade *anything* else, [italics in original text] like "immunity" from paying rent if a traded property is landed on, or a promise not to build houses in the future. (Chp 2, The Rules Explained) - All trades are based on assets owned at the time of the trade. No options or immunity from paying future rents may be granted, nor may partnerships be formed. (Chp 6, Tournament Monopoly & How You Can Play It)We've chosen to play this way and it speeds up the game both for the reasons outlined above AND because it makes trading simpler - since you have to trade real property, everyone can make clear decisions and valuations about what & what not to trade.
Hope that helps - and while we're at it, let's remind everyone of the four most-commonly missed rules in Monopoly (the quotes are from the official rules): Free Parking:
A player landing on this place does not receive any money, property or reward of any kind.This is just a "free" resting-place.Property Auctions:
If he does not wish to buy the property it is sold at auction by the Banker to the highest bidder. The buyer pays to the Bank the amount of the bid in cash and receives the Title Deed card for that property. Any player, including the one who declined the option of buying it at the printed price, may bid. Bidding may start at any price.Building Shortages:
When the Bank has no houses to sell, players wishing to build must wait for some player to return or sell his/her houses to the Bank before building. If there are a limited number of houses and hotels available and two or more players wish to buy more than the Bank has, the houses or hotels must be sold at auction to the highest bidder.Loans:
Money can be loaned to a player only by the Bank and then only by mortgaging property. No player may borrow from or lend money to another player.