Mike H,
As you say, hitting 300 yards and averaging 300 yards are two very different things. I hit it a long way, but I'd never in a million years claim to average 300 yards....it only takes one ugly heel hit that smothers in the left rough to really mess up your average. Not to mention that hitting a half dozen pretty good but not quite square drives and/or having a 50% or less fairway average don't do much for you either!
However, I don't think average drives are what's important here. Just because I can't average 300 doesn't mean I can't easily hit and exceed 300 yards on a well struck drive in normal conditions (no wind, level ground, not PGA tour fairway height and dry but not fast n firm conditions) Its not so much the fact that a 540 yard hole is a drive and a 3i for me every time, because that's nowhere near the case, I'm not even close to consistent enough. Its the fact that it CAN be, whereas 10 years ago it was only after 2-3 weeks with little or no rain on when I had one of those nice summer southerlies blowing 20 mph behind me that I could do that.
In a way, I think its made me worse on par 5s. If I step up to the tee of such a hole now, I feel a little extra pressure from inside to hit a nice solid drive, and keep it in the short grass, so I have a chance to have a go. Hell, there's one 590 yard hole with an uphill second that I think about when the wind is blowing because one glorious day (with a nice breeze, of course!) I pulverized my drive and hit a pure 1 iron that was a thing of beauty to behold. I loved the feeling of getting to the top of the hill and seeing my ball on the green, and want it again. Its like a drug for some of us, one we know is bad for our golf score but we don't care!
That said, I have to temper this by admitting that my home course has a 494 yard par 5 that's downhill for the first 330 yards and uphill the last 100, but overall downhill. And plays with the prevailing wind. And I'm only on that green about 1 out of every 7-8 times, because the landing area is narrow and hump backed and I'm just not that straight with my driver on a regular enough basis to take advantage of it. Hell, for a season I played it with a 1 iron off the tee because my driver was bedeviling me and I eagled it more times that season than I had for the previous two or the three since, how sad is that?
So whether its a problem for guys like me depends on your point of view. I'm capable of making a lot of par 5s play differently than they were intended to be played. But in general, being able to do that will probably make me play it worse, so it certainly isn't leading to lower scores. Personally I think it is more of an issue on formerly long par 4s like 450 yards or so that are a driver/wedge on a good day and but always reachable in two even after a bad drive -- used to be that a missed drive on a hole like that meant I'd be either hitting the hero shot or thinking where I should lay up to.
Oh yeah, good example of all this, on #12 Lahinch a couple weeks ago, I pounded one 385 off the tee (nice breeze, natch) Left with a SW in, but that's one crazy green and not easy to hit, so I put it in the back right bunker and took a 5. Same score my dad got, after he hit a good drive and a good second, about 25 yards beyond my drive....which he had to point out as we walked off the green...