Kev, I think you're thinking in terms of photography as many people do. longer exposure time in practice can reveal more distant (ie faint) objects in the signal. Stacking improves the SNR which gives you extra flexibility for teasing out faint signal from the data before the noise overwhelms it. Each source frame I take is rarely exposed anything like what I end up with, which is what you aim for in photography. The way I approach my deep space AP is capture as much signal as I can and understand what various tools I can use to make best use of the signal. There are some amazing things possible with PixInsight such as removing the stars from an image so you can work on the faint signal to make it visible then add the stars back. Most astrophotography people talk about are just photos, they rely on someone's personal opinion of what the image should look like and may not be accurate to be able to take scientific measurements.
In any image you capture there is a lot of signal that may not be apparent or wanted. Increasing exposure time will get you more signal (even more of the signal you aren't aware of) but the quality of the signal is also effected by tracking quality to avoid smearing it. You can of course over expose and reduce the usable signal. More frames can improve the SNR which can give you a cleaner/smoother image of a well exposed target but also let you do more processing to bring out fainter signals. It all depends on what you are trying to show with the shot. One of the things I enjoy with my AP is just exploring the data, I plate solve everything and see if anything of interest is within the field of view then I can try to reveal the faint fuzzies. Since my stroke a year ago I can't use my telescope and my AP is mostly just with a DSLR on a tripod, or a high end "compact" on a vixen polarie and I'm constantly amazed at what I can capture. The Southern Pinwheel Galaxy is one of my most proud shots using DSLR +500mm lens and its not visible in any of my shots, but once registered and integrated there is plenty of signal to pull out the spiral galaxy. If you try to make the most of your capture hardware and have a bit of an understanding of your data and what you want to achieve with it you will be amazed at what you can get, buying more gear might not be the answer to improving your imaging. I'm no expert and plenty of people with better gear and good understanding make better shots but then again I've seen plenty of shots from people with more expensive gear that are just garbage compared to what I can achieve.
So i guess, exposure time vs more subs = depends
