http://www.gpoaccess.gov/help/congress_table.html
http://en.wikipedia.org/wiki/United_States_Congress#Procedures_of_Congress
The original intent of this constitutional provision is to prevent the other states or their legislatures from detaining the representation for the people of the other states from being able to do their duties. This was a very real concern as the British had done just that in the recent past and it was still fresh in the minds of the framers of the constitution. The fact that they mentioned it at all in such an important document shows just how important it was to them. That they then clarified what was considered a justifiable reason for detainment and that no other reason was allowed should have also clued you in to their intent.
Keeping in mind the conditions and methods of transportation available in 1788 (no trains, planes or automobiles) it was of national importance to allow the free passage of all of the members of congress in order to ensure that they could perform their duties. It's not like they could catch the next plane and skip a layover in order to make it back to the Capitol on time or make a phone call saying that they were going to be a little late. A long delay might have serious consequences in the conduction of the nations business.
Now then, with that being said I wouldn't be surprised at all if there have been times in more recent history where a congressperson has been detained for reasons not technically allowed under the constitution who
didn't exert his privilege to free passage under the constitution. It's meant to be used for the reasons I laid out above and it would be very bad publicity if they did for less than a very good reason.
However, they chose to not do so (like Rand Paul did here) either because they felt that it wouldn't actually have any effect on them being able to do their duties or because they would only draw even more attention to their dalliances and follies or for some other personal reasons.
That doesn't mean that they
could not have exerted those privileges given to them by law.