To support my contention that it is a legalistic matter (and even then, only after the
serving of US geopolitical interests), more than "what PH brings to the table", that
produces the contrast in US positions on the KIG dispute versus the Senkakus':
No, the US Won't Back South Korea Against Japan on Dokdo
http://thediplomat.com/2014/05/no-the-us-wont-back-south-korea-against-japan-on-dokdo/
As the Korea Herald opinion pieces mentions, a consequence of the United States standing by Japan
on Senkaku is that South Koreans are beginning to feel that the U.S.-South Korea alliance plays
second fiddle to the United States’ much more developed alliance with Japan. If this feeling persists
or proliferates in South Korea, the United States will find itself needing to do a lot more to “reassure”
the Koreans of Washington’s commitment to the alliance.
What SoKor "brings to the table" as a defense ally/partner, is significant,
both in terms of military capability, and as a geographic asset to US interests.
However, although the SK-US MDT was already in "final" form by January of 1954,
and was ratified the month after, it was nearly a year before it was actually put
into force (only on November 17, 1954).
The US had
first required the attachment of an 'Understanding', specifying that:
"It is the understanding of the United States that neither party is obligated, under
Article III of the above Treaty, to come to the aid of the other except in case of an
external armed attack against such party; nor shall anything in the present Treaty
be construed as requiring the United States to give assistance to Korea except in
the event of an armed attack against territory which has been recognized by the
United States as lawfully brought under the administrative control of the Republic
of Korea."
http://avalon.law.yale.edu/20th_century/kor001.asp
Combine the ^above-highlighted with the Van Fleet Report submitted October 4th
of 1954 (Gen. James Alward Van Fleet, as President Eisenhower's special envoy,
had been sent to the region to develop such reports, partly dealing in anticipated
territorial disputes between would-be US partners/allies), which says of "Dokto" /
"Taka Shima":
http://2.bp.blogspot.com/-PqBNyIUzJ_Q/UDSsaVen4hI/AAAAAAAAAVw/2QbvFkYK7tY/s1600/Ownership+of+Dokdo+Island.jpg^That throws doubt on the LEGALITY of SoKor possession of Dokdo/Takeshima
(at least from the US point of view, and within the narrow context of Van Fleet's
interpretation), despite SoKor actively claiming possession of the islands at the
time of the inking and ratification of the SK-US MDT.
Japan wasn't even actively-protesting SoKor's claim at that time, yet the US was
already anticipating that Japan would have legal grounds to do so, in the future.
There's your 'legalistic', for you.
Contrast this to KIG.
Unlike Dokdo/Takeshima, which SoKor was actively claiming possession of, at the
time of their MDT-signing with the US in 1954, KIG wasn't even being claimed by
PH when it was sealing its MDT with the US in 1951.
The KIG wasn't considered to be included in PH territory under the MDT, because
KIG wasn't yet considered PH territory by PH itself at that time... so how the hell
could the US ever be expected to be as vocally-protective --under MDT terms--
of PH claims to KIG, as the US has been of Japan's to the Senkakus?
PH could first legalize its sovereignty over KIG, then the US might retro-interpret
the MDT to cover KIG. Or not, given the current risk (to the US) involved in taking
such a stance. Regardless, the US had been urging PH to take its claim to court,
but for decades, PH has dragged its feet (until very recently), for reasons that
are too nauseating (especially these days) to repeat for the nth time.
PH claim to KIG is quite strong, founded on patent
res nullius, and on UNCLOS.
This claim however happens to post-date the MDT, hence the US position on KIG
is publicly one of neutrality, aloof of its ironclad MDT with PH.