I think it is user preference.
Professional athletes will have different needs to amateurs.
On one hand people claim that they are unable to swing a 350g racquet for a full match as they get to tired.
On the other hand these same people would have you believe that they are able to swing a 300g racquet at full speed with perfect technique for an entire match.
These two positions are unbelievable.
A professional athlete has the choice which way they go. A desk jockey what isn't fit enough for one, definitely is not fit enough for the other.
At some point of tennis growth, a player develops his strokes using certain specs. It's logical you feel the best when playing with similar specs racquets, as your strokes are modelled using those specs, so they will likely be the most effective using those specs.
I started with rec tennis as a student in early '90's when there was lots of heavy racquets around and when SW around 330 was normally present in rec racquet selection as well, not just players racquets. I developed my strokes using heavier racquets and though those strokes were nothing special in terms of bare RHS and not quite technically sound (especially FH was not), I used my core on both FH and BH as this was an easy way to get some power and consistency when wielding a heavier racquet. I was self taught, never studied strokes, using the core came naturally.
Now, if a player learns his strokes using a light racquet and is self taught like I was, few things will likely happen. One is that he will develop a high RHS since this is a must when playing a light racquet based on player's head size. The second is that he will not quite need using the core to get this high RHS since racquet is easy enough to swing to get lots of RHS from your shoulder rotation, so he will likely use his core less. And of course, his strokes will get naturally completely accomodated to those specs. Now, if you give such a player a heavier racquet, it's normal it won't work. He will try to reach the same RHS using mostly his shoulders, and doing this will wear him out.
But there's another aspect. Heavier racquets are sensitive to weight distribution, and weight distribution is a matter of manufacturing variance, which can accidentally happen to be better or worse. If worse, you get a stick that really is laggy and tiring to swing with. It's not about its SW as much as about something wrong with the (non existence of) equilibrium of its weight distribution. In which case this racquet is simply hard and inferior to play with. Also, when there's a customization involved, customization itself can mess the weight distribution up if not done the right way. Result is the same (sometimes even worse) than a bad manufacturing varation racquet.
I have some custom setups that have heavy specs but are quite easy to play with (say, around 355 grams and 350+ SW). I had different players tried them. None of them complained racquet is hard to swing with, many were surprised when I told them the specs. They found it surprising they can actually swing them easily. Some complained there was too much power on them, however. And it's question whether they could play better or even equally good using them since those specs are not what they're used to playing with (though most of them would not get tired since racquet is carefully balanced to be easy to swing with). And even for me it takes adjusting to make the most of them, since I grew on playing with SW approx. 330 and 340-345 grams racquets (it was mostly Prince Precision Spectrum 630 MP 97'' 14x18 with double basic grip and OG).
in my beer.....Da GF just called me a dinosaur.... 
