The way I like to think of items in general is this: Cheap items are good for the money, but take up a lot of item slots, legendary items are great for the item slots, but relatively bad for the money. To use the Dorans vs BF sword example used earlier, A BF sword costs 1650 and gives 45 AD 3 Dorans cost 1425 and give 30 AD, 240HP and 9% lifesteal it also leaves 225 gold for health potions, mana potions, or wards, ad 25 gold more could let you grab an elixer to try and turn your lane quckly and safely, meanwhile, what will 25 gold on top of a BF sword buy you? Nothing. That being said, should you buy six Doran's items? No, unless you're getting forced out of lane so quickly that you only have about 475 gold each time you go back, you should not be buying too many Doran's items, for 3 reasons.
Reason 1) Doran's items do not build into anything, which means their value doesn't get paid forwards towards something bigger
Reason 2) You only have six item slots, which means once you have six, you can't itemize further unless you sell, which leads onto...
Reason 3) Doran's items only sell back for half what you bought them for, this means half your gold spent on Doran's are lost forever, where most items sell back for 75%, or 100% of their value can be put forward towards an item they build into.
For this reason, Doran's items should be used for getting a quick pay-off, either in gold for your team, or negative gold for the enemy team. So unless you plan on using your Doran's item to try to push your enemy out of lane, push down a tower, get a kill, or keep up in a situation where you'll fall behind without the great per-gold value of a Doran's, it's probably better to pick up something else, as something else will scale better into the late game, where a Doran's is what it is now, and will be less later.
I do feel, however, that there are many better choices for item builds, and seeing boots+3 on my entire team is usually very painful for me. Seeing it on the enemy team usually makes me very happy. Boots+3 pots provides you with some minor regeneration, and vital move-speed, but it leaves a serious gap in terms of stats or real sustain. I find 4 things to be vital in the very beginning of laning phase, those three things are effective farming, denying your opponent's farm, staying in lane and, by extension of 3, avoiding death. Boots help you avoid death, and can be marginally helpful for farming, denying farm, and staying in lane, but only marginally, and not in any way that learning good positioning won't. When I play, for example, an AP Mid, I take a Meki Pendant and 2 potions, either HP or MP or one of each depending on my champion (ex. 2 mana pots with Fiddlesticks, 2 health pots with Annie). The Meki Pendant means I'll almost never have to leave lane because I'm out of mana, and allows me to be more aggressive as a result (my opponent will never get a break to go farm freely because I'm low on mana, as long as I don't waste it on nothing), and the potions allow me the sustain I need to stay in lane Hp wise. This setup helps with denying farm, because you have mana with which to remain a threat, helps farm effectively, because you get more frequent skill uses to farm, and the zoning makes you safer, helps with staying in lane because of the mana sustain, and helps you avoid dying because you have skills available that you might not have had if you didn't have the mana regen from a pendant. While Meki+2 is less effective then boots+3 with direct help in surviving ganks, positioning and map awareness can keep you very safe in lane.
While I am aware this is a guide for newer players, I feel that taking boots+3 too frequently negatively impacts your play because it allows you to slack on map awareness and positioning, and I feel that being ready to go without boots until your laning opponent is forced to go back to base is a great learning experience just as much as it is an advantage in lane.