Firstly, it was done historically. The well-known "poem of the pell" from the late 1400s suggests using a wooden "mace" of double the weight of an actual sword to train swordsman for strength as well as skill, emphasizing delivering powerful (though presumably not wild uncontrolled) blows:
Quote: |
And, as it were, upon his mortal foe:
With mightiness the weapon must be cast |
It is, of course, a rough paraphrasing of Vegetius's writings on training roman army recruits a millenium earlier (and Vegetius in turn, cites this practise from even earlier sources). I suspect that the fact that the wooden weapon was double-weight may also increased its strength somewhat; long ago I broke an ash waster on a pell with an over-enthusiastic zornhau.
Secondly, you will probably want a waster or plastic equivalent at some point anyway if you continue to practise HEMA. Many (though not all) groups use them for drills and/or sparring. They do have limitations, both in realism of the simulation of the weapon they represent and safety, but they are a lot safer than sharps for any sort of partner-drills, relatively cheaper than a blunt trainer and can even be constructed by oneself if one has some woodworking ability.
Thirdly, while Craig's points regarding nicks and edge damage are valid, you can probably avoid some of the worst damage during the early stages of learning by first getting used to swinging a waster or even an appropriately sized stick around, gradually replacing it with the actual sword as you gain skill and confidence. Edge alignment is obviously important, but for some reason it didn't seem to bother instructors in the period when swords were actually used; see, for instance the "mace" in the poem of the pell, or the round cross-section single-sticks which were considered good enough to train for broadsword/cutlass/sabre in the period of their actual use on the battlefield and in duels. I know that one napoleonic era sabre manual suggests practising cutting on vegetables, and I've read of eastern-european saber-fencers doing cutting practise on clay or straw figures, i.e. separate cutting exercises were done (not unlike japanese tamashigiri) so perhaps that they did similar exercises for this purpose in earlier periods as well.
Interestingly, a friend who practised kenjitsu (not kendo) was advised by his instructor to practise striking hard blows with a bokken against a car-tyre hung from a tree and free to swing. (NB. This I would definitely not do with a sharp sword; I know from personal experience long ago that hitting a tyre with a machete (and so presumably a sword) can cause the weapon to rebound unpredictably if technique isn't perfect, and even if it is, you run the risk of messing up your cutting edge quite badly on the embedded steel wires inside most tyres. With a waster it would be safer.)
Anyway, these are just my thoughts on what I think is ultimately a question of personal preference (within certain logical boundaries)