2001 325Ci tire questions

My 2001 325Ci convertible comes with Michelin MXM 91W tires in size 
225/45R17.  Car still has OEM supplied tires except for two replacements
due 
to tire damage.  Car mileage is 39000 miles (low).  I just had my 3rd tire

fail.

These are low profile tires and I have had unexpected loss of air a total
of 
3 times in the life of the car.  Each time the tire was destroyed and have

been replaced twice.  They have been replaced with the OEM type tire at a 
cost of $275 a pop.

Two questions:

1) Why are these tires being damaged like this?  Is it the low profile?  
Is 
air pressure critical where sidewall wear can result if the tires are even

slightly underinflated?  I think folks with low profile tires like these
can 
help answer this question.  To my knowledge there have been no pothole or 
curb encounters.

2)  I believe that if I replaced all 4 tires at the same time, there is 
likely a tire that is much better than OEM, and at the same time more 
cheaply priced, that I could buy.  By having only one tire to replace at a

time, I feel I am captured into buying the OEM type tires at a substantial

price.  Comments?  Are there other tires in this size that are less 
expensive with equivalent or better performance for the simple grocery 
getter driving that I do?

Thanks for any and all replies.

Vito