We don't use our greenhouse and bulb house for growing seeds. These are grown outside as they are all seeds of alpines. We do use them for alpines that can't take overhead wet, bulbs in pots so we can see them flowering earlier, growing on seedlings after they have germinated... Really depends on what your gardening interests are.

I agree with Moongower. I would fill a greenhouse with cacti but they are not to everyone's interest.
You could grow tomatoes or peppers etc in it ...
You can also use a greenhouse for overwintering plants that would not survive outside in the frost etc.
It also depends on where you site the greenhouse - in sun or shade etc ...

In your blog"hang on to your plants" you related you were getting 80mph winds at you coastal area residence. If you do decide to get a greenhouse make sure that you won't have to hang onto that too.....so you might consider getting suggestions for what type of greenhouse would be best to withstand those conditions.

as already said, growing tender plants, a collection of plants that wont survive outside, over wintering and then veg .

if you don't plan any of these activities then it is expensive for seedlings. I'd use the space for another flower bed/feature.

You need to ask yourself not what you could use a greenhouse for but what do you want to grow that really needs a greenhouse. Why not get one of those wooden framed mini greenhouses that stand against the house wall and see how you get on with that? I daren't have a greenhouse here because of the winds but find one of those very useful. (One of my neighbours lost his altogether last week and he's not the first...)

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I start seeds off with an electric propagator in front of the garage window and then put them in the conservatory so I don't have to heat my greenhouse. But then they end up in the greenhouse when it's warmed up. You can produce lots of bedding and tender vegetables, you will need to bring them on in the greenhouse. You can grow tomatoes and peppers, melons and the like to fill it. Can overwinter tender plants, take cuttings and pot on shrubs and divide herbaceous subjects. The shelter allows you to garden even in the rain; it can be your little sanctuary. But beware, there is a downside; once you get one, it will never be big enough and can you trust your neighbours to look after your treasured plants when you go on holiday?

The greenhouse is great in the winter for plants that need to be frost free and then in the summer you can grow tomatoes etc.

Like a lot of folk I think its tomatoes and peppers inthe summer, and at the mment its home to all the shrubs in pots that can't take the frost......my geraniums are all still in flower, its a windy hill, cold when the east wind blows......in the winter of 2010 the prolonged frost even froze the pots but thats hopefully rare, the summer smell and taste of home grown fresh toms is worth all the effort