DIY-capable homeowners do their research.  They select the best products, they shop for the best prices.  Certainly warranty of the underlying material is part of what drove you to selecting your roofing materials.  A new roof is a significant investment with a long life (30-50 years on asphalt shingles).  A good warranty could save you a bundle, in the event there are problems in the future.
 
However, the manufacturer’s warranty only covers manufacturing defects in the roofing material itself.  Architectural shingles may well include a Lifetime limited warranty from manufacturing defects to the original homeowner.  This may even be transferrable to the next subsequent homeowner (if you sell your house).  However note that this is shingles only, many other materials (read the under the shingles series) do not have the same warranty.
 
More importantly, this warranty doesn’t cover two (2) items:

1. Installation or workmanship
2. Labor to replace the defective shingles

 
Asphalt roofing companies operate high-technology manufacturing plants; the risk of manufacturing defects is very low; hence why they can offer generous warranties.  At the same time, manufacturers are very aggressive in inspecting the roof prior to providing warranty coverage and have identified that most causes of “warranty” claims are due to the installer’s workmanship, and not the material itself.  I have seen situations of delaminating shingles where the manufacturer simply dropped a handful of new bundles on the driveway and left.  So while you may think you have limited-Lifetime coverage, you may have nothing. 

What can you do to protect yourself?  As a professional roofing contractor in business for over 20 years, and with intent to stay in business and service my customers longer, I provide warranty coverage against any errors in workmanship.  In addition, I am “Installer Certified” for several lines of asphalt shingles that I install; the manufacturer’s warranty includes a reasonable labor or installation cost to repair or replaced manufacturing defects in these situations.
 
One of the biggest motivations of DIY is saving money.  Make sure you understand the limitations of the manufacturer’s warranty.  More importantly though, make sure you don’t try to spend dollars to chase dimes.  By that I mean, don’t try to save money on your roof by DIY if you think you will have warranty coverage later.  And with your house as your most significant investment, do you really want to risk your first line of defense against the weather to trying to save a few dollars?  You likely have other home improvements you would like to focus on; leaving roofing to the roofing professionals will enable you to undertake these other projects, and know that you have warranty coverage for installation and workmanship errors as well as labor in the situation of manufacturer’s defect.

One of the biggest luxuries DIY-motivated homeowners have is time.  You have a day job, your DIY home improvements are your hobby.  This is something you enjoy after work, on weekends, during your free time.  Your home improvements are something you take great pride in doing, doing well, and showing off to others.
 
But let’s think about this for a minute.  How many of these projects have a critical time component associated with them?  You have a great bar in your basement, what are the consequences if you got sick in the middle of the job and didn’t finish it on time?  What about your fantastic man cave – you know who you are?  Or remodeling that guest bedroom?  If these took a couple of weeks extra, was it really that big of a deal in the scheme of things?
 
Your roof – you don’t have that luxury.  Once you start your roofing project, you need to get it finished.  Even if you fall into the thought process that roofing underlayment provides you weather protection until you get shingles installed, let me assure you that this weather protection has a maintenance price.  Every rain you will be inspecting your underlayment.  Moderate winds will render it useless and any wind-driven rain will result in roof leaks.  Roof leaks result in more sheetrock repairs, which as a DIY-capable homeowner you have already proven that sheetrock is a project that you have no fear of.
 
This is one of the greatest factors if you consider a roof a DIY job.  When professional roofing contractors start your job, it is going to be finished quickly.  And it isn’t just nailing those asphalt shingles up to finish the job; those shingles have a glue strip that needs to set before your roof is truly weather tight.  Once you start a roofing project, you want to make sure it gets done and the shingles properly glued => before the next rain.