The first obvious answer would be: iOS and Webdirect (WD) don't work very well with plug-ins. Okay, that is a very very good reason, but what if iOS and WD are of non importance to you?
Over the past 16 years (give or take 1 or 2) I have used plug-ins from Troi, CNS, Dracoventions, myFmButler, 360Works, Productive Computing, New Millenium, Goya, 24U, Dacons, and MonkeyBread. (and even a few others, but they aren't active anymore)
Pro: Operation that normally are possible, can be done with plug-ins: special dialogs, file-manipulation, decent emailing, connect to other software, crud-sql,
Pro: all of the plug-ins have proven to be very reliable
Pro: When issues came up during development, the vendors supported us very well. (okay, maybe one or two didn't)
Con: certain plug-ins only work in specific versions or a specific range of versions of FileMaker.
Con: some plug-ins are not available cross-platform
Con: sometimes within a plug-in certain functions aren't cross-platform
Con: some plug-ins can only be registered when there is a web-connection
Con: sometimes the registration is tied to an FM-installation
Con: when a vendor goes OOB: no more updates will be issued and eventually you must seek a replacement for it
I did not write "the costs" in con's. because plug-ins to me are simply function-sources and thus timesavers. The time saved outperforms the costs many times. Not only when building, but also when problems do occur, an easy integration of a functionality makes for solving trouble much faster.
My most important pro is that often plug-ins made things possible, that could not be done in FM, like doing crud-sql, listbox-dialogs, manipulate files etc. Some of the functions provided by plug-ins where, over time, partially, replaced by functions in FM, so sometimes I discontinued using a plug-in.
Today I still use a bunch of plug-ins and their version-maintenance is usually pretty easy .... except for the plug-ins based on Java and .net, they often require extra attention.
For me plug-ins are not a problem and I like using them. In fact the way I work/program has the use of plug-ins integrated and I use a library-file for easy integration of them into my solutions.
Here in the community I often see others going out of their way to avoid the use of plug-ins and I just ask myself why? Sure there are challenges regarding iOS and WD, but besides these, I wouldn't see much problems. So I am curious for other thoughts on this in the community.