In the old days (before the FIA changed it in 1996) the team who won the world championship took the numbers 1 & 2 from the team who previously held them (unless they already held them of course). The team who had been 1 & 2 before would then have to take the numbers from the team who had just won the championship.

When Alan Jones won the 1980 championship Williams took 1 and 2 from Ferrari and so Ferrari had to take the 27 and 28 that Williams had raced with in 1980.

Ferrari subsequently held 27 and 28 until 1990 when Prost arrived as world champion. Therefore they had to take 1 & 2 and McLaren had to have 27 and 28 which is why Senna was 27 for 1990.

For 1991 McLaren took 1 & 2 back as Senna had won the championship in 1990 so Ferrari had to take 27 and 28 back. They didn't have them through choice. It's just the way it worked back then.

Ferrari then held 27 and 28 until 1996 when Schumacher arrived as world champion. Ordinarily, Benetton would have been obliged to take 27 and 28 under the old rules but the FIA changed the numbering system for 1996 onwards with cars being numbered sequentially with no gaps, thus 27 and 28 fell into disuse and have not been used since.

Excluding Indy 500's, number 27's first race was the 1952 British GP where Johnny Claes stormed from 23rd on the grid to finish 14th in his Simca-Gordini (you remember). Its final race was the 1995 Australian Grand Prix where Jean Alesi qualified 5th and retired on lap 23 after colliding with Michael Schumacher.

The only way 27 will come back is if the number of entries increases significantly since under the current numbering system there simply aren't enough cars to justify its inclusion.