All depends on how Blue Magic is implemented. If spells are easily accessed, and learn rate is 100% (like it is in every single FF game except for FFXI's BLU, and this one), the only excuse for not having a spell is because the player didn't take a few minutes to go and learn it. Learning Blue Magic doesn't HAVE to be a difficult and time-consuming process, and I don't believe it enhances the enjoyment of the job when it is. I certainly don't feel enriched by the seven-plus hour session I spent butting heads with Ifrit, back before you could learn it from Ifrit Normal.
In such a case, exclusion due to not having learned a spell would happen only to the most extremely lazy of Blue Mage players. We're talking a level of laziness far in excess of, say, a WAR who never learns Steel Cyclone because they're too lazy to do their job quests - and I doubt there's a player here that would have a lot of sympathy for a WAR THAT lazy! Learning BLU spells could (and in my opinion should) have been as easy as tracking down and fighting one or two of the appropriate mob, or possibly running a dungeon or trial exactly once.
No, concern about Blue Mages being excluded from parties for not knowing a spell is as much nonsense as Yoshi P's other excuses. Whether it's actually a problem or not depends on how the job is implemented, and it doesn't take a lot of imagination to find plenty of ways to prevent it from being a concern. Folks that criticize the "party exclusion" excuse are NOT kidding themselves.
The bottom line is that Yoshi P had a vision for Blue Mage that was frankly incompatible with the job structure in FFXIV. He felt he had to choose between compromising on his vision to make the job fit, or adding the job as a side activity that kept it safely removed from the game's major content. He went with the latter. The thing is, though, that Yoshi P's vision is only one interpretation of a job that has had a TON of variance over the history of the Final Fantasy franchise. Basically the only things that all Blue Mages have in common are that they use monster abilities and they have a collection of abilities to fill out. The latter is pretty flexible as to just HOW they learn the spells, as well - Quistis, for instance, could learn Bad Breath without EVER having fought a Marlboro (all you need is the drop, and she doesn't have to be in party to get it). Even the way we DID get BLU takes advantage of this flexibility - several of the spells learned are learned by consuming items!