Pro: you don't waste one disk sector on a partition table. (Yay.)
Pro: the disk can be used in an operating system that doesn't support PC-style partitions. (Like you're going to use one.)
Con: this is unusual and may confuse co-sysadmins. (See?)
Con: if you install another operating system, it might think that the disk contains garbage and make it easy to accidentally overwrite it by selecting the wrong disk — whereas operating systems generally leave alone partitions whose type they don't understand.
Irrelevant: extending the filesystem is not easier if it's directly on the disk than if it's in a partition, nor vice versa. (Being on LVM would make it easier.)
Conclusion: it works, but it's not a good idea.