Portable Ops actually is part of the storyline, bro. Albeit in a small role.

OT: If Kojima's more heavily involved with this title than MGS: Rising, and if this is the missing link between Portable Ops and Metal Gear 1 (in other words, the building of Outer Heaven), then I think it's safe to say this is the true MGS5.

Spin-offs are usually part of the main story, but small parts that isn't necessary to be known to understand the main story

You have a point.

I see spin-offs as having a story that intersects with the main (series) story. Since in this case, the story in Portable Ops is very much of the main MG story, I can't see it as a spin-off.

Yes. If the story's there and Kojima says so then I don't see why not.

Kojima's word is all that matters.

Well Kojima himself said he thinks of Peace Walker as a MGS5. If the creator of the series says its really a MGS5 then who is anyone to question him. Also MGS Rising is being made by a younger team and is most likely going to be a hack n slash type of game. How would a game like that be any type of sequel to the MGS series?

Kojima said, I believe.

And Rising is a action game like Devil May Cry and not a MGS like... so Peace Walker is MGS5.

Metal Gear Solid: Peace Walker was supposed to be called Metal Gear Solid 5: Peace Walker. That's about it. According to Kojima-sama, it's going to be a true successor to Metal Gear Solid 3. The fact Kojima-sama himself is working on Peace Walker instead of Metal Gear Solid: Rising explains enough. Metal Gear Solid: Peace Walker is a true sequel to the series.

Portable Ops felt the same as Metal Gear Solid 2: The Sons of Liberty, a spin-off, albeit MGS2 brought more to the continuum of the series. Nevertheless Portable Ops is a part of the main series.

I was a bit turned off after hearing about Co-op in a Metal Gear game, but it's the same with the multiplayer in Uncharted 2 At first I was like WHY!? but then I felt like why wasn't there a multiplayer in the first game! After playing the demo I feel that co-op might just be the best thing ever happened to Metal Gear.

Portable Ops is canon in the MGS Database as a part of the main story arc, even if it is a minor one.

Peace Walker is essentially the direct sequel to MGS3/prequel to MG in telling the series of events leading up to the creation of Outer Heaven. From an overall story arc perspective, it's a significant part.

Presumably, there will some day be a game titled MGS5, although with the story of Solid Snake finished, it would have to move in a different direction. The main conflict and story arc was closed and there really isn't any motivation for another mission unless they want to make MGS 5: Retirement Ops.

Rising sounds like it may be another story gap filler in explaining the events alluded to in MGS4, directly following MGS2, rather than another story that happens after MGS4.

Not much point in quibbling over which game deserves or should have had a number in its title.