Yes, this is almost certainly a bad idea.
The bladder is located in a very sensitive part of the body, and by holding in urine past the point at which your body wants to remove it, you're increasing the pressure on a number of organs and body parts that probably don't appreciate it. You can cause yourself a great deal of long-term complications which can manifest themselves in any number of ways: damage to the bladder muscles and ligaments, problems with the bowels, problems with the prostate, and probably worst of all, problems with the kidneys.
The reason for this is that all the time you're holding in the urine, your body is still producing more, and if it has nowhere to go, the excess urine can start to force it's way 'back the way it came' and back up into the kidneys. This is a very dangerous situation, as not only are you increasing pressure on the kidneys, but you're forcing waste products back into the kidneys, and potentially even into the bloodstream - you're putting yourself at risk of kidney infections, which can be potentially life threatening, and also permanent kidney damage, which could mean you're letting yourself in for a huge, huge number of complications and problems.
Also, by holding in urine past the point at which your body gets the urge to void, you're basically collecting a liquid inside you that's a perfect breeding ground for lots of nasty little bugs and germs - and it's a fact that people who empty the bladder less frequently or improperly are at a much higher risk of contracting a urinary tract infection (a UTI). And those can be very painful, and have the potential to again cause serious long-lasting damage.
Potentially, but if you drink excessive liquids, you might just wake up part way through the night and then have trouble:
a] Wetting whilst lying down, or wetting whilst in bed (I know this has been a problem I've encountered personally)
or
b] Relaxing and getting back to sleep, especially if you're having a hard time sleeping anyway.
You might want to consider some other options: there are hypnosis tapes/CDs available that are supposed to promote night-time wetting; how much you're willing to invest in these (both financially and in terms of the success you expect to generate) is up to you, but it might be worth a shot.
One other suggestion is maybe try and get hold of some authentic 'Dandelion and Burdock' drink; not the carbonated kind you get at a supermarket, but the authentic type from a health food shop. Dandelion is a natural mild diuretic, and by drinking some before you go to sleep, it can induce nocturnal urination. It is so well known for this property, that the French have a name for the plant ('Pisenlit') that translates as "Piss-the-bed". But make sure that you're only drinking as recommended by the manufacturer, and not excessively. Also note that as it's a natural 'remedy', it's effects and effectiveness will vary from person to person - so don't be too disheartened if it doesn't actually work for you.
Rawr. x
PS - Please note, though I do have some medical training, nothing in this post constitutes actual medical advice. Don't use the internet as a substitute for proper health care! If you're worried about anything, or you're considering taking a herbal remedy, speak to your GP or a trained healthcare professional, especially if you're already taking prescribed medication.