I have to say I agree with Campguy here - reading the advice re. not using guyropes on tent made me cringe - sorry!
I too work in a camping store (and I am an experienced camper as well, although working full time in retail can make it difficult to get as much time in the bush as I would like
) - I've been in the industry for close to 10 years (and a camper my whole life!) and I manage a store for an independent chain. We have a manufacturing and repairs facility as part of our business and the amount of tents I see week in week out that are destroyed due to the guy ropes not being used is just phenomenal.
Of course there are exceptions to the rule in everything and some people will be lucky enough to camp for years and never even break a pole. I personally used to have a couple of good quality dome tents that lasted for a good 15 years regular use and saw some pretty severe storms without a hitch.
Unfortunately, in this day and age, not everything is made to last, and most items are made to a price. Most manufacturers will cut corners where they can - this is a fact of life... If they can get away with using slightly thinner poles they will - the guy ropes are there, in part to compensate for this.
The guy ropes on a tent are designed to support and add stability to the frame. They help to prevent pooling of water under heavy rains, the weight of which can otherwise cause damage to your tent, and they support the frame in winds. There are very few tents on the market that are designed to be set up without guy ropes (external frame canvas cabin tents & canvas, sprung-steel frame domes are a couple that do not require guy ropes)
Most tent warranties do specify that they will be null and void if the guy ropes are not used - it is fairly easy for an experienced tent sales or repair agent to pick if the guy ropes have been used or not, and tent wholesalers are generally quite tough on this point. We can tell by the way a pole breaks, wether it is from excess stress or wether it is a manufacturing fault.
Most of the time the damage is minimal - i.e. a broken pole or two, easily fixed, but around 20-30% of the tents we see that have not been guy roped have more severe & expensive damages such as large tears where the fibreglass poles have snapped and torn through the fly of the tent.
One of the most important things we stress to all our tent customers is that they should use the guy ropes at all times. After all, if you are buying a good quality tent then you probably want to ensure it stays in good condition for years of use, and if you go for the budget end of the market, then I would advise that using the ropes is vital to support the structure of your tent.
Also - the bigger the tent, the more important it gets to use all those ropes... A large tent can catch a lot of wind and act just like a sail.
P.S. I saw a comment on this thread from Derek about staff at camping stores giving bad advice. Unfortunately this is often the case - the amount of mis-information handed out is scary. You have a better chance of recieving the right information if you visit the specialist retailers rather than the department store type places. Specialists generally pride themselves on giving the right advice and selling quality branded product that they believe in, rather than flogging their own cheap imports at any cost. Just my 2 cents worth...