I quickly googled the subject and found that, actually, in common Slavic period all verbs had distinction between "concrete" and "indefinite" verbs (each action had two similar verbs, one for "general" action, and one for specific one-time action). Then, it transformed into perfective/imperfective "pairs" of similar verbs. That is, for most of them. Yet for verbs of motion there's still a distinction left between action in general and/or repeated and/orin several directions, and action as a specific act of movement to a certain destination once.

1. What we have with unprefixed verbs: a pair like ходить/ идти, носить/нести, плавать/плыть. An indefinite verb and a concrete verb.
2. With a prefix: I tried some verbs... It seems that only "по-" makes a perfective regardless of the verb. Like походить, пойти, поносить, понести, поплавать, поплыть. Note, that for concrete verbs you get the meaning "started doing action", but for indefinite the meaning is "to do it for some time and then stop".
For any other prefix yo just get an aspectual pair: indefinite verb (ходить, носить, плавать) remains imperfective, while concrete one-directional verb (идти, носить, плыть) becomes perfective. So, you have imperfective/perfective pairs like приплывать/приплыть, приносить/принести, уходить/уйти, подносить/поднести, забегАть/забежать ("забЕгать" has different meaning - "to suddenly start running around"). Maybe, my despise for sentences such as "Она приносить ему полотенца" comes not from grammar, but purely from the fact it's rarely used this way. "Она подносит полотенца" seems fine. "Уважаемые пассажиры, поезд прибывает на конечную станцию X" too ("Dear passengers, out train is arriving to terminal station X "), though, "прибывать" isn't a verb of motion, so that doesn't count

In other words, I don't care what's written in Rosetta Stone - but in real speach "Я несу/ Ты несёшь/ Она несёт/..." is a completely normal way to express being in a state of er... transporting some object.

As for the hints... let's face it, all the examples are rather artificial. "Я несу тебе кофе" is the most natural, roughly translating to "I'm serving you coffee/ I'm bringing you coffee". I mean, right now. Might be (theoretically) an answer to a question "What are you doing?" Anyway, in real life situations people tend to say enough to be understood. This "number of times" and directions only describe general guidelines as to which verb you should use to express a certain idea the way Russians do it in their laguage. Which idea to express - that's up to the speaker. But yeah, "несу кофе" is mostly for "right now" and "приношу кофе" usually describes a repeated activity. If the verb isn't a verb of motion - say, like "Я рисую" - "I'm drawing" -, it may be ambiguos. However, if you were to hear this as an answer to "What are you doing in arts school?" - it'll be pretty clear that the action is habitual rather than one-time.