In many ways the Pink form is a Phoenix replacement, just without the cosmic baggage or the interpretation that it is a separate force/entity/being. It allows her to power/level up to or near her foe's power-level, she draws on the psychic energy of others (similar to how the Phoenix Force has been said to draw from the psychic energy of every living being and/or future unborn), and it has allowed her to become a being of pure psionic energy and reconstitute herself (this parallels the original explanation for Phoenix given in UXM 125) when she beat the Poisons and effectively resurrected herself. She has also used it as a power/threat/dominance display similar to how she has used the phoenix raptor before. Also learning about Phoenix/Dark Phoenix and being shown footage of Dark Phoenix in Trial of Jean Grey allowed her to look in herself to discover this new application of her power to begin with.
Since she absorbed some of the Phoenix Force's energies with it in Generations, she can also create a pink phoenix raptor (of course Adult Jean could also create a pink psychic raptor). So really, with the pink form she doesn't even need the Phoenix Force anymore. She can have its best qualities without the baggage. Claremont was going to make her a Phoenix on her own without the Force in Revolution and now they could really do it with Teen Jean's development. Past-Adult Jean even called them "two Phoenixes" with her having the Phoenix Force and Teen Jean having her Pink form in Generations Phoenix. I doubt if they will continue this direction though. They never do what makes sense with what they have set up.
I agree that ground rules would be good and it should be something used from time to time and not a constant. But if Angel can power up to Archangel, I don't see why Jean can't power up to Pink mode. Maybe a limit could be that she can't sustain the pink form for very long or that it taxes or hurts her body. Both Nate Grey and Xorn-Jean grew so powerful that their powers were killing their physical bodies so maybe the threat of that could limit Jean. Her power levels could also be limited by who she is around or fighting against. Maybe if there aren't that many people around and/or the mutants around her aren't that powerful--maybe she doesn't get that much energy.
This is kinda unrelated but here is a pic of Marvel Girl-Jean from the original days levitating multiple steal beams (from [Uncanny] X-Men #36). Angel acknowledges that Jean's powers have grown. Although Jean admits some strain it does show that she wasn't as weak as some think back then. Her growing powers was always a part of her character too.