The baseline karma gen rules are, IMO, completely broken without implementing some houserules. They make humans worthless, and encourage you to play trolls and orks with low body scores...and the rules completely shaft awakened folk. If you GM decides to use those rules without modification, tell him you'll be playing a free spirit

Anyways, with the basic BP system, the costs of things are a bit different than the cost of things with karma. To start, a BP is generally worth approximately 2 Karma. This is why, for instance, it is inefficient to purchase specializations during creation, as 2 karma is not very valuable, but 2 karma is halfway to another skill rank 4. BP costs also are uniform while karma costs scale. For optimal build efficiency with the BP system, you will have attributes(and skills) that either have a high or a low rating, as a mid rating is a waste of resources(it costs 25 karma to raise an attribute from 1 to 3 with karma, but it costs 55 karma to raise an attribute from 4 to 6, you get a better karma to BP ratio by using high attributes).
The spirit issue mentioned is actually a bit more simple. Karma and BP are very important resources for awakened characters, especially magicians. Everything that increases magical power, even foci, requires karma or BP, and many magicians will find that they don't have a big use for money. That is why binding spirits is so powerful, because it's one of the few advantages a magician can get that costs only money(and some time and some drain). BP spent on spirit services could just as easily be spent on spells or skills or binding foci. The fact is, unless your GM is very stingy, you'll get more nuyen then you need as a magician, and have plenty of mats for binding spirits, but you'll always be karma starved.