Just out of curiosity: would you allow a player with Unseele magic to use it just like thaumaturgy? It came along like that in the OP. I ask because my view seems to differ a bit from that notion.
For me sponsored magic is like picking up evocation and thaumaturgy rather than channeling and ritual but with it's own limitations that effect it in different ways. One of the limitations is, as zenten responded, a thematic limitation. Another is the sponsor's agenda.
I would have a hard time with someone using unseelie magic to actively and directly benefit summer. As the fae can only work within their natures (and as their natures are to struggle against their counterparts) I have a hard time with power that is essentially an extension of one of the fae working against it's own nature. In effect (since the strengthening of summer is the weakening of winter) the power would be harming itself, lessening itself as it worked toward summer's agenda. Of course someone could work towards summer obliquely, helping them "by accident."
The last limitation, which is both a limitation and a benefit is the whole sponsor debt/compels.
An Example to clarify: Should the Summer Knight with his Seele Magic be allowed to do a tracking spell on just anyone?
I would say yes. As there is no real way of telling what may or may not benefit winter's agenda I'd give a character a pretty long leash. Of course it would also depend on the character. If they've fallen out of favor or hindered something in the past then maybe the fae trusts him less, and may put further restrictions on the power. As long as the character is in good standing with his sponsor then I'd say the sponsor likely trusts him to work toward their interests without constantly monitoring every little thing the character does and approving or vetoing every use of power.
Edit: Then again it would have to fit within the baliwick of the sponsor. I could see a "tracking spell" of a winter emmisary being flavored as calling up some fairy that tracks them for you or tells you where they've been. Of course if your sponsor is, say, the Erlking then it's probably much easier to explain a tracking spell within your baliwick.