The purpose of the mash is to convert non-fermentable starches into fermentable sugars.
The base malt (which forms the basis of any mash) contains enough enzymes to fully convert it's own starches, and usually has enough enzymes to convert some adjuncts (other products that contain starch but no enzymes for conversion).
The problem comes when you increase the amount of adjuncts beyond the level that can be handled by the enzymes in the base malt. Once you reach this stage, you will be adding starches to the finished brew for no increase in sugar yield.
Most malts are two row malts which contain enough enzymes to convert themselves, but leave very few left over for converting adjuncts. Conversely, 6 row malt (grown only in America) has a higher concentration of enzymes and can convert more adjuncts. However, 6 row malt has lower yields than 2 row.
As for the boil, this has three main purposes.
1. To break down the proteins, causing them to precipitate out (hot break and cold break)
2. To extract the bitterness from the hops
3. To drive off unwanted volatile compounds like DMS that could adversely affect the flavor of the finished brew.
As for adjunct flavor, adjuncts have very little if any flavor, and I think that the vast majority of brewers would agree that it is better to add more malt, rather than adjuncts to increase yield and flavor. The commercial brewers that use large amounts of adjuncts probably do so for cost reasons, but for home brewers, adjuncts are not cheaper than malt.
I hardly ever use adjuncts, but when I do, I keep the amount down to less than 10% of the grist (except for stouts). This allows the adjuncts to provide their benefits (such as improved head retention with flaked barley) without causing conversion problems.
Wheat malt is a base malt, and can be used without problems in small amounts. In larger amounts, it tends to produce cloudy beer, which is fine for some beer types, but I don't like cloudy pale ales.
-a.
