Not such a crazy idea for smaller leagues perhaps..
for eg.
In the A-league (australia, poor league 1 or 2 style football)
there is a salary cap in place 1.25mill pound for the 23 man squad (designed to stop teams going bust or unfair domination)
the exceptions are
Marquee spot - 1 player, who must meet FFA approved standards that can be paid outside the cap, no limits.
Youth Marquee- 1 U23 australian player to be paid up to GBP 75k (a year) to prevent talent drain.
Injury replacement- if a youth player is unavailable to replace longterm injury, a player outside the cap is allowed (loophole)
and then the relevant one
The guest player. - any player who will raise the profile of the game at any price, for up to a 9 game spell..
This is where the OP has a point.
what is better? a half-fit, old injury risked and heat affected Robbie Fowler for the 27 game season
OR
up to 9 games of the season played with a player of the calibre of Tranny or Messi??
Realistically, players of that calibre won't come at any price, but look at Eto'o with his little loan to Budonyakor in Uzbekistan.. It really is a possibility, and for smaller leagues in countries where football is trying to gain profile (Oz, USA etc) it would be a huge bonus to the season..;