I'm making a late hopped extract beer later tonight. (Jamil's Evil Twin as posted http://www.mrmalty.com/late_hopping.htm) He suggests at 90 minute boil with the first hop addition coming 20 minutes before the end of boil. Since I'm doing the extract plus grains version of it, is boiling for that long necessary? Why or why not?
Typically, the long boil is intended to increase melanoidin formation ("kettle carmelization") and decrease DMS in wort with a lot of pilsener malt. The former appears to be the case, here:
"While you could go with a shorter boil, the 90 minute boil enhances the blood-red color. It also adds a touch more melanoidin and caramel notes."
It's not necessary, but it will add some nice flavor and color characteristics that you might not get otherwise. The sugars in the extract will still darken and go through the maillard reactions, just the same as an all grain version.
I always did at least a 1hr boil when doing an extract brew (with various hop additions). It can take ~20min. for the hot break to occur, depending on the amount of protein in your extract. It is recommended that you wait until the hot break occurs before doing your first hop addition and timing an hour. In your case you should be safe adding your hops after the hot break, boiling for 20 additional minutes, then cooling, etc.
According to How to Brew, the 1 hour boil is really just for the purpose of the hop editions, and you wouldn't need to boil at all if you're using pasteurized hopped extract, because the boiling is just for pasteurization in that case. My guess is that the 90 minute boil is for the hops, just as Dogfish Head makes 60, 90 and 120 minute versions of their IPA. You need the longer times to get more hop bitterness and flavor - but those beers are all "continuously hopped".
It is not at all necessary to boil the malt extract "wort". It is only necessary to boil the hops. The hops can be boiled in water alone for the required 60-90 minutes or whatever time is needed for the various hop additions used. Once the "hop tea" is ready it can be strained into the fermentation vessel and rinsed with boiled water. If worried about sterility, one can dissolve the malt extract in just boiled water and add to the fermentation vessel - otherwise just pour it in and rinse can. So, boiled "hop tea", dissolved malt extract and whatever sugars are required are all in the fermentation vessel. Then add cold water to volume. Towards the end keep an eye on "wort" temperature. Below 15 degrees is not so good for pitching yeast and neither is above 30 degrees. 20 degrees is about right. When the mixture is good to go - pitch the yeast as normal. Even with a 90 min hop boil it should not be more than 2 hours from start to end.
If roasted grains are used in the beer for colour and/or flavour then steep the grains in (for example) boiled water for between 20-60 mins and strain the grain into the fermentation vessel after the malt and hop tea.
Not boiling the malt is a safer and cheaper process to make fine beer.