The only bummer with letting your pass completely expire (past 30 days), is that you have to get a new card at the processing center the next time you come.
Take me for example. Pass expired in December, I didn't renew. Now I buy my new pass and I'm going this weekend March 30th. I gained 3 months, but this weekend, I'll have to stand in line and get a new card. Not that big of a deal, but I am driving up Saturday and I expect to stand in line for about an hour waiting to get the new card because I won't get there till the afternoon.
Plus I'll end up parking across the street from the garage because I don't think they'll let me into the parking lot from other posts I've read. I'm not gonna drive up to the parking booth, and have them charge me $8, saying I can get it back later, when noone has, when I'm only going there Saturday to renew the pass and do a couple rides.
So it depends on when you are coming back again, and if you want to stand in line for a new card.
Now, had I renewed my pass in December, I would have lost 3 months, but wouldn't have any delays this weekend. I can deal with the tradeoff of having to park across the street and wait an hour for my new pass on Saturday because I'm spending a full day on Sunday plus Monday morning.