The two-day flights to the ISS were done for a couple of reasons:
First, it allowed the freshly-launched crews a couple of days to get used to being in free-fall. Roughly one person it three suffers from
more-than-mild "space adaptation syndrome" (SAS). Nearly everyone gets the "full-head" feeling from their blood redistributing itself evenly
throughout the body (as opposed to concentrating in the lower-body, as it does in 1G). Added to that, the body's disorientation as it gets used to
the idea of "no up" can cause dizziness, nausea and vomiting.
These symptoms manifest more strongly and quickly if the newly-launched astronaut moves around a lot. In the early days, Mercury & Gemini astronauts
did not have room in their cramped capsules to move around, so SAS was not significantly observed. Starting with Apollo and continuing with the
Shuttle, astronauts could get out of their seats to do work. This brought-on more severe cases of SAS. On the Moon-missions, this was OK because the
3-day trip to the Moon allowed time to adapt. However, on the Apollo 9 test flight of the Lunar Module in Earth orbit (a busy and demanding mission)
Rusty Schweikart's SAS caused delays and modifications to the flight plan.
One of the early (and on-going) objectives of the Space Shuttle was to study SAS and ways to alleviate it. Drugs such as "scop-dex" (a mix of
scopolomine & dexedrine) helped. As a "common sense" measure, the affected astronauts tried to limit their movements, especially of their heads.
Like so many things in space travel, "common sense" turned-out to be wrong. It turned-out that limiting the onset of SAS actually prolonged the
adaptation-time. Nowadays, in addition to medication, astronauts have a series of head-movement exercises to help get through the adaptation period
more quickly - but it still can take a couple of days. The goal is to have the astronauts ready to move around and get to work as soon as they get to
the Space Station.
The other main reason for the two-day flights to the ISS is that it allows more flexibility in the launch _ Anyone who followed the Shuttle
program could tell you that the complex beast rarely launched on-time. However, if you're not in a rush to make rendezvous, then you can have several
minutes in each ISS-pass in which you can launch. If you're reasonable certain you can launch on-time (which is not terribly difficult with
less-complex launch vehicles), you can launch into a closer orbit to your target and thus reduce (and even minimize) rendezvous time.
For the Apollo missions, they wanted the Lunar Module ascent stage taking-off from the Moon to rendezvous as soon as possible with the Command Module
that would take them home. From the outset, this is what they practiced. Gemini 6 made the first-ever orbital rendezvous in December, 1965 after
four orbits. Its target was the Gemini 7 capsule, already in orbit on a long-duration mission. Three months later, Gemini 8 also did a 4-orbit
rendezvous and performed the first docking (with an unmanned "Agena" target vehicle). That summer, Gemini 9 made a 3-orbit rendezvous. Gemini 10 did
a modified 3-orbit rendezvous, then undocked, changed orbit and rendezvoused with the old Gemini 8 Agena. Gemini 11 topped them all by rendezvousing
with its target on the very first orbit after launch. For the Apollo missions, they originally planned for 3-orbit rendezvous, but with the
confidence acquired from the Gemini missions, they were able to use 2-orbit rendezvous on the first lunar landing missions, and then 1-orbit
rendezvous on the later missions.
So, in summary: Manned spacecraft are capable of making orbital rendezvous fairly quickly after launch, but for a variety of reasons, they usually
take more time.
Hope this helps.
edit on 29-3-2013 by Saint Exupery because: paragraphs are our friends.