Ok so a couple questions...
First, how is it "promoting" passenger play to let the character gain Xp for planning and such and then sitting back?
Easy answer: most players (and characters) when given options will, all else being equal, trend toward the path of least resistance. Here, that path is to sit back and let others take the risk.
The character gains the same Xp after planning and such if they go join in the action as if they sit it out - so it seems it would neither promote or discourage either choice. *IF* your character wants to go join in the "derring do" great, if not, just fine. Seems like if the character has an investment in-game in the outcome and has skills that can help, its smarter to go and help, but hey, maybe not. meanwhile, if the others are fine with this, what is the problem?
The character who rarely if ever joins in the derring-do is, by law of averages, going to outlive the rest of her party (and probably their replacements as well) by a wide margin; all the while slowly but surely accreting wealth and xp the others don't have.
More to the point, how would the game be "better" if the "passenger" did all that, stayed back, let the others go do the derring-do and was just gradually lower and lower level compared to them?
is it going to be much more awesome funtime for all if at "third tier" the passenger is say 9th level or even 6th level so that when the team is working all as a unit in scenes he can provide less?
Eventually one hopes the passengers will realize why they're falling behind and join in more often.
What I've seen happen in the past goes roughly like this. This is an abbreviated version where things happen quickly, to show my point; but imagine this spinning out slowly over a 20-adventure campaign:
Party forms with 6 characters A-B-C-D-E-F, let's say character A is the passenger. They go out in the field and after their first adventure their status is:
A - alive, full share of xp and loot*
B - alive, full share of xp and loot*
C - alive, full share of xp and loot*
D - alive, full share of xp and loot*
E - dead
F - dead (this, IME, would be my guy)
* - loot includes possessions of dead companions E and F, along with what the adventure gave.
So, a couple of recruits G and H come in to replace E and F and off this meery crew goes. After their second adventure they look like:
A - alive, full share of xp and loot (so now has 2 total shares of each)
B - dead
C - alive, full share of xp and loot (so also 2 of each)
D - alive but missed half the trip, so half share of xp and loot (thus 1.5. total of each)
G - alive, full share of xp and loot (1 total of each)
H - alive, full share of xp and loot (1 total of each) (my replacement for F - made it through an adventure, yippee!)
And they're all richer by sharing out B's stuff.
So now character I comes in to replace B, and off we go on adventure 3. Results afterward:
A - alive, full share of xp and loot (so now 3 and 3 overall)
C - alive, full share of xp and loot (so now 3 and 3 overall) - but at this point C retires, having had her fill of all this.
D - alive, full share of xp and loot (so now 2.5 and 2.5 overall)
G - alive, full share of xp and loot (so now 2 and 2 overall)
H - dead (I lose another one...)
I - alive, full share of xp and loot (so now 1 and 1 overall)
And a fourth adventure includng new recruits J and K is all we need to make my point clear - off they go, back they come:
A - alive, full share of xp and loot (so now 4 and 4 overall)
D - dead
G - alive, full share of loot and xp (so now 3 and 3 overall)
I - alive, full share of xp and loot (so now 2 and 2 overall)
J - alive, full share of xp and loot (so now 1 and 1 overall)
K - alive, full share of xp and loot (so now 1 and 1 overall) (my third try at this...)
Character A, by simply being a passenger while the rest of the party took the risk, is now both the wealthiest and the highest-level character in the group; with wealth even further enhanced by sharing in the loot from her dead companions. Assuming each adventure produced a vaguely-equal treasury, A might be 4 times as wealthy as J and K after the 4th adventure. Yet A has achieved this by intentionally sitting back and letting others take the risks, sometimes to their demise.
The level difference can be mitigated by the DM allowing new characters to come in at a particular level relative to the party average or whatever...but the wealth aspect cannot, as treasury division is almost universally a player-driven element. The DM can introduce new characters with some possessions, sure, but has to be wary about those possessions simply adding to the wealth of others when the new recruit dies.
second, how is this passenger thing a problem if the players are enjoying the play? How is that so much of a problem to warrant some form of level skew being a good penalty?
The passenger player might be enjoying it but the rest of the crew might very well end up feeling resentful, particularly if my example above starts coming out real.
Lanefan