ISO vested but underwater - any disadvantage to exercising and holding?

[hbregman1]

After reading a bit I was unable to find an answer to this. I have ISO that are vested but not worth anything now. Seems like you only pay taxes on ISO when you sell. 

What if they stay underwater for the duration of their lifetime after you exercise? 

Any tax consequences? 

If not, what is the disadvantage of exercising and holding? 

Thanks 

[Critter]

WHY would you ever  buy something now for a $1 if  you can buy it for 75 cents instead ?  Never overpay for something just because you can ... there is no benefit for doing so.