Wheel spacers accomplish the same thing as a wheel with less backspace. The advantage of spacers is you can have a different offset on one axle compared to the other while still keeping all your wheels and spare interchangeable.
Increasing the offset by either reducing backspacing with a wheel or spacer or significantly increasing the wheel width will put a greater load on the wheel bearings. When the weight and forces of the vehicle work farther outboard the bearings, there is that much more leverage on them.
On the front axle, steering is also affected. Although the wider track width can allow greater steering angles and smaller turning radius, excessive offset can also increase steering effort. See scrub radius:
http://en.wikipedia.org/wiki/Scrub_radius
With a small increase in offset, the effect may be neglible. A little more and you may see premature bearing wear. Anything extreme is ill-advised.
The best solution is to lengthen the axle. This requires a complete axle swap or the lengthening of the axle housing and half-shafts. It's been done both ways. I understand that Ashcroft's Force 9 axle is lengthened compared to a stock Rover axle. I also read that Keith at Rovertracks has extended some axles, I assume Rover housings with Toyota internals. I'm sure others have shortened portals and tons to something a little wider than stock. With a wider axle, you can have the track width while keeping the load more or less centered on the bearings and a negative scrub radius.