Yes, denied creeps still give experience to opposing heroes, although not as much as if they had died another way.
From the wiki:
A denied creep divides 36 exp to enemy heroes within 1300 range. A melee\ranged creep would normally give 62\41 exp in 1300 range respectively.
So denied melee creep only give the enemy 58% of its experience bounty, and denied ranged creeps only give 88% of theirs.
Some abilities are exceptions to this. Lich's Sacrifice splits the full experience between all heroes on both teams in XP range. Some abilities that consume creeps can deny all experience.
Based on experience and gut feeling: no, this is not a viable strategy, except perhaps in extremely unique situations. In pro play the effects of a player leaving will always be negative.
In lower level play there may be a few other things to consider why it'd be beneficial to have teammates leave:
- The player was feeding, leading enemies towards fellow teammates, or in other similar ways making the opponents stronger;
- The player may've been drawing teammates into useless chat (flame wars, etc), where they'd otherwise be doing something beneficial;
The latter actually happens a lot in low(ish) level play, I see it a lot at 3K MMR. Sometimes a player leaving brings a sort of calm to your team allowing you to focus and win after all. This is especially true if leftover players can still fill all essential roles for endgame play (control, carry).
Finally, about your last question: "Is it allowed in tournament play?". I'm no expert on official rules, but I imagine most rule sets are similar to this one from joindota, with these relevant sections (emphasis mine):
5.3 Disconnects & Reconnects
If a player disconnects, the game is to be paused instantly. Everyone has up to ten minutes to reconnect to the game (if the same person disconnects more than once, all times get added up). If this time is passed, the opposing team may continue the game, unless an admin demands the opposite. The affected player is in this case still allowed to rejoin later on.
and
5.7 Ending
A game is finished, when an Ancient Fortress is destroyed, when one team obviously forfeits, when the majority of a team leaves on purpose, or when the administration decides on it.
These rules don't really "forbid" leaving as a strategy, but from the tone you can kind of guess that it's not even remotely considered as a viable strategy.
Basically your question is similar to:
Are soccer players allowed to score an own goal to throw off their opponents mentally?
I guess it's "allowed", but the rules won't say much about this situation as there's no real way anyone'd consider it a viable strategy.
Best Answer
No, if you are just disconnected, your gold will still increase and won't be splitted and shared with your team mates. In the same fashion, while you are disconnected, they won't be able to drop or sell your items.
However, the moment you abandon a game (either by not reconnecting for 5 min or by voluntary leaving) all your gold gets divided amongst remaining team mates equally, and your items can be sold. The gold from the sell is also divided and shared. From that point on you still get gold over time, but any time the amount can be divided by the number of remaining team mates (if 4 players remain, every 4 gold acquired; if 3 remain, every 3 gold acquired; etc.) it will be divided and each member of your team gets 1 extra gold.